Copy and paste the url, then add [tube ]... [/ tube] tags (with no spaces) immediately around it (make sure that it has an http at the beginning of the address and that there are no spaces). For instance (without the spaces this would embed):
[tube] http://www.youtube.com/watch?v=pZdlm8LWxmM[/ tube]
Alternately, click on the full reply editor

Then click on the Insert Movie
icon, then add the youtube url to YouTube File Name (think you still need the http:// in the address.
By the way, you should always double-check to see if the embedded video works since some youtube vids have embedding disabled.
Edited by Logan - September 11 2012 at 15:23