When you click the youtube button above the smile face you will get a <youtube></youtube> in between those you put the regular youtube link and it will embed it in the forum. Except, instead of <> it'll be  (couldn't figure out how to get it to ignore it). Or you can just type it in but remember to use brackets.
E.g., here is my youtube link: http://www.youtube.com/watch?v=JDXdOdZ-Cqw
Here is my embed code <youtube>http://www.youtube.com/watch?v=JDXdOdZ-Cqw
With appropriate brackets , it is this: