What you do is, Post Message, "Go Advanced", click the YouTube icon and paste just the code from the YouTube URL in the box (usually the part that comes after 'v=' but before '&'). Then you should end up with something that looks a little bit like this:
Code:
[YOUT*BE=CflblizqpC8][/YOUT*BE]
Only it says YOUTUBE instead of YOUT*BE (I had to do that to get it to show the code).