I didn't see any other questions like this on the forum except when someone didn't turn on "Include Magic the Gathering" filter on character sheets. 

I purchased Curse of Strahd from the Marketplace and can't find or add the NPCs, monsters, or items to the campaign. I am able to find the book information through the compendium, but none of the other purchased, associated items. 

Any assistance or pointing me in the right direction would be greatly appreciated! Thank you!

I'm not sure what you mean by "add the NPCs, Monsters, or items to the campaign."  There is not a way to add NPCs, Monsters, or items to the campaign page.

Do you mean you can't add them to the encounter builder?  Or that you can't access the monsters (for example) under Game Rules>Monsters?

If that is the case there are two possibilities I can think of:

-you inadvertently bought the compendium only version, and not the full source.  

-your entitlements need to be resynced

The latter can be addressed by checking on your username in the upper right corner of this page, choosing account, and the en marketplace tab.  Look under "Account Entitlements."  If it says you need to sync, click the sync button.  

While on the page, scroll down to the section that says "Current Product Licenses" and look for your recent purchase.  I think it will indicated whether you purchased the full source or just the compendium.  I've never made a compendium only purchase, so I don't know for sure how it shows up in that area.

Got it, thanks. I didn't realize you couldn't add them to the campaign page. This was really helpful.

Thanks for clarifying. What you can do is use tool tips in the DMs notes section of the campaign to add links to such things. for example: “six orcs lie in wait in this room”

see this thread for more info on tool tips.