I guess this is the right section for this. What  I am trying to do is add a multi-class to an existing character. I have what just became a level three monk, I want to add a 1st level bard, so it would be a half monk half bard. I know that I've read somewhere on DnD Beyond that multi-class characters are allowed. Is it that I can't add to an existing character, is it that I can't use those particular classes. Or am I just not doing it right?

From the home page I click on "My Homebrew Creations",  Then fill out all the boxes, and click Filter, and the page just blinks, but nothing happens. Not even an error message. I've also tried to click on "Create a Sub-class", That let's me create a subclass from scratch, but not using an existing character. So I don't know what to do. 

Any help you can give me would be greatly appreciated. This is my first time using DnD Beyond so keep that in mind and make your instructions very simple. Thank You so much for your help. I hope I explained this correctly so you can understand it, if there are any questions just let me know. 

Multi-classing means that when you level up, instead of continuing in your current class, you instead gain a level of a different class. You do not need to use the homebrew tools to do this. I assume you just hit 3rd level, and currently have two levels of Monk. By multiclassing, you would be a Monk 2 / Bard 1. You would NOT have 3 levels of Monk. Note that you will need a Charisma score of at least 13 to multiclass into Bard.

to get to the edit character screen:

My Characters -> Click on "EDIT" below the character name

Then on the class tab, click "Add Another Class" at the bottom

Then click on Bard and Add Class in the popup

That method would add to the current character, instead of creating a new character. I am asking because when I go this route it just seems that it is creating a new character not adding to a character, but I could be wrong. 

I think that Houligan has laid out as clearly as can be how to multiclass using the dndbeyond interface. If you are more generally asking about how multiclassing works in the 5th edition ruleset, check out Chapter 6 of the PHB. When you level up, you can either (A) go to the next level of your current class, or (B) add a level in a new class, if you meet the stat requirements for multiclassing that your current class and the new class provide. For example, if you are currently a level 2 monk, and want to add a level of bard when you reach third level (to become a Monk 2/Bard 1), then you will need Dexterity and Wisdom of 13 or higher (monk requirement), and also Charisma of 13 or higher (bard requirement). After doing that, you are a third level character, but you have the features of a second-level monk and a first-level bard. That means you need to look at Chapter 1 for your proficiency bonus based on your character level, not just trust the proficiency bonus value that is printed in your class table(s). 

But again, dndbeyond does all that math for you. Just follow Houligan's advice on how to add the second class to your character and dndbeyond will do the rest :) 

No, I get all that, I understand the rules now, and yes Houligan did an excellent job. My question is about the computer system of adding a character. When I try to add a class it seems the computer is creating a new character instead of adding to an existing character. I hope that is clear, I know sometimes I don't make things clear. Just to restate my question, it seems the character roll up system wants to create a new character instead of adding a multi-class to the existing character. 

I think you'll find that after using the "add another class" button you're able to on the dropdown define how many levels you have in each (monk 2, bard 1), and when you then click on "view sheet" you'll see everything merged as one Monk 2/Bard 1 character. 

The method that I posted will add onto an existing character. You will notice that, per the multiclassing rules, under Bard, it will not grant you the extra saving throw proficiencies, and you only pick one skill, rather than the three that a new Bard would.

Thank you both I really appreciate it.