So, I have a player who wants to be a member of the Witchlight Carnival. There's a background for this, and I do own the book, but for some reason this isn't showing up in the character creator for her, even after joining the campaign.

Is there a place I can go and select which sources I can allow for a specific campaign, and if so where is that?

Nope, everything is automatically shared once Content Sharing is enabled.

• Is the player’s character actually in the campaign in which you are sharing content? It must be to take advantage of your content sharing.
• If the character is in that campaign, then you and your player should go here and make sure your Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace)
• If that still doesn’t fix it, try turning your Content Sharing off and back on again.

I've tried the others, how do you turn off Content Sharing?

Quote from Lastsonofzod >>

I've tried the others, how do you turn off Content Sharing?

 
The same way you turn it on for the campaign. It isn’t an automatic thing, it must be activated individually on a campaign by campaign basis:

Given your question I feel the need to ask, Do you/someone in the campaign have a master subscription? If not then you cant share official content within Dndbeyond.

I'm having a similar issue I have content sharing enabled but it's only showing the basic stuff and none of the stuff I have bought can you only share your purchases in one campaign? Because I have another one on going with some friends but another group have approached me hoping I DM a beginner run

You can share content with up to 5 campaigns at a time, and if anyone else has a master tier and activates content sharing in other campaigns you are in your content would be shared in those as well. Theoretically you can share content with an infinite number of campaigns as long as enough people activate content sharing for all of them.

• Are those other players building/editing their characters specifically in the campaign in which you activated content sharing? They have to in order to access that content. (Content is shared within the campaign, not with the players themselves.)
• If that’s👆not the problem, you and the others should follow this link and make sure your Account EntitlementS are in sync: (https://www.dndbeyond.com/account/marketplace).
• If that’s👆not the problem, you should try turning content sharing off and back on again for that campaign.

The link says it's all working I've tried turning it off and on again for some reason it looks like I don't have any extra content to share

I'm going to post a consolidation of stuff related to this issue, please forgive me if you've tried some of these things before.

1. D&D Beyond is a digital retailer and tool set. They are no different from your local bookstore. In order to have access to the content on D&D Beyond you need to buy it through D&D Beyond or make a duplicate within the homebrew creator. For more information, look at these threads: Hardcover Books, D&D Beyond, and You -  D&D Beyond vs Physical Books: An Explanation - Homebrew FAQ - Everything You need to know about Homebrew - 
2. I bought the cheaper version. In addition to being able to read the books on D&D Beyond you need to buy the digital tools. There is an option for ‘Compendium Only’ content with each book purchase that is usually ⅔ the cost of the whole book. That content does NOT give you access to the digital tools usable in the character builder, but does give you access to read the book. If you only own the compendium content you'll need to buy the integrated tools to be able to use that content on the character sheet. The best way to check this is to go to the market page for the book and see if you have the checkmark next to any of the stuff aside from what is labeled 'Compendium Only' at the bottom of the page. The cost of any individual part of a book gets reduced from the cost of the whole book, so if you did buy the compendium content the cost of it would count as credit toward the purchase of the book, so you won't be buying it again if you bought the digital tools through the purchase of the book.
3. I bought the digital tools on D&D Beyond, but still don’t have access to it. Check to make sure that you are logged into the correct account. It's easy to accidentally have more than one account because of the varying ways to log into the website. If you do find that you have more than one account, you should open a support ticket to get them merged by clicking here or via the ‘Help Portal’ link at the bottom of the page.
4. I don’t/sometimes have access to the stuff. The content on D&D Beyond is shared with a campaign, not you specifically as a player. You need to be making the character within the campaign to be able to have access to the tools if they are being shared with you. The same thing goes for homebrew made by other people.
5. I’m in the campaign and still can't use the tools. Content sharing needs to be enabled within the campaign by someone with a master tier subscription. This person does not need to be the DM, just as long as someone has content sharing enabled everyone's content will be shared throughout the campaign.
6. Do you have the content toggled for your character sheet?  Make sure that the slider is red (to the right) to have the content enabled. Linked here is an article written by the staff of D&D Beyond about how the sliders work and what each slider actually enables. It is important to note that content is toggled based on the book it originally came out in, and not necessarily the book purchased from. This is important with much of the content from Tasha’s Cauldron of Everything that were originally published in the varying Magic the Gathering books. Again, see the article about it linked above.
7. Make sure that you and your players have their entitlements in sync. You can go here to ensure that all content you should have access to is being properly mapped to your account. If you don't want to follow the link you can go to the drop-down menu that pops up when you hover on your username at the top right of the page and click Account->Marketplace. You should see a section near the top labeled ‘Account Entitlements’ that will tell you to sync your entitlements or that your entitlements are in sync.
8. Make sure you bought the player race, not the monster. If you bought something specific from a book it's important to know if you bought the player race or the monster. There are several player races that are also monsters, and it's easy to accidentally buy the Changeling Monster and not the Changeling Race as an example.
9. Do you meet the prerequisites? There are prerequisites for many of the mechanics in D&D that are toggleable on the first page of the character sheet with the other toggles. If it is toggled on (Red/to the right) then you need to meet the prerequisites to be able to use the content. You can always disable the toggle for prerequisites and see if the content reveals itself.
10. I bought it through the App. The connection between the app and the character builder often takes time to update. Since the payment has to go through the app store and then to D&D Beyond there is an entire other system that needs to get checked and verified before the content will show up. D&D Beyond doesn't get the information regarding your purchase until it has gone through the app store. If you find yourself having issues with the app, you should open a support ticket by clicking  here or via the ‘Help Portal’ link at the bottom of the page and reach out to the app store you made your purchase through for more information regarding your purchase.
11. Force the system to update. Sometimes the system is finicky. You may have to “wake up” your character sheet to force it to update its cache. Add and remove Inspiration or equipment and refresh the browser page. You can also try having the person with the subscription toggle content sharing off and on again. If that doesn't work try using a different device/browser to look at the content. If the content was recently purchased or it is homebrew that was recently updated it can take a bit of time for the content to become available to you because of server update latency, usually 15-20 minutes.

Thanks for the help guys, my problem was that I had more than five campaigns active. I shelved a currently dormant one and it's working properly now.

for some reason none of this has helped or worked I have bought a lot of content through dndbeyond and I am incapable of sharing in this second campaign I've started

At this point, it would be worth submitting a support ticket via the help portal link at the bottom of the page.

Question... If I have content I want to share but don't want to dm, can another player access my materials to build their campaign? Does just one of us have to have the master sub or both or...?

Quote from Undead_DaVinci >>

Question... If I have content I want to share but don't want to dm, can another player access my materials to build their campaign? Does just one of us have to have the master sub or both or...?

If any one person in a DDB campaign has a Master Tier subscription and activates Content Sharing for that campaign, all content owned by anyone in that campaign will be shared with everyone in the same campaign. The person who activates Content Sharing need be neither the content owner, nor the DM.

Related question I guess I'll put in here; is there a way to stop my homebrew content being shared in someone else's campaign with content sharing enabled? I'm a DM for a campaign that uses a bunch of my homebrew content, so I can't remove it from my collection, but with content sharing enabled in another campaign I've joined as a player, the other players can seemingly see all my homebrew spells and such and its causing a lot of confusion. The DM also uses their own homebrew content so players can't just turn off homebrew on their character sheets either.

Quote from TheTak >>

Related question I guess I'll put in here; is there a way to stop my homebrew content being shared in someone else's campaign with content sharing enabled? I'm a DM for a campaign that uses a bunch of my homebrew content, so I can't remove it from my collection, but with content sharing enabled in another campaign I've joined as a player, the other players can seemingly see all my homebrew spells and such and its causing a lot of confusion. The DM also uses their own homebrew content so players can't just turn off homebrew on their character sheets either.

 
Content Sharing is not required for the sharing of Homebrew, all homebrew is always automatically shared within campaigns for free, even without anyone in the campaign even having a payed subscription at all. The only way to not share homebrewed content is to remove it from your collection.

Seems quite inconvenient that there's no setting for that, hopefully one is coming in the future.

Quote from TheTak >>

Seems quite inconvenient that there's no setting for that, hopefully one is coming in the future.

 
Friendly advice: don’t hold your breath. 

I'm sure its coming, along with that homebrew revamp and homebrew base classes, right? Right??? :)))))

Right…. 😂😂