By default, only the admins can embed YouTube video code. There is a way to allow regular forum members to post Youtube videos, see bottom of this posting.  While logged in as admin, you can embed the code for video (like the code you’d get from youtube.com if you uploaded your video there) like this:

[html]

embed the code here

[/html]

To allow regular members to post youtube video, you can add the YouTube BBCCode Mod.