If you want to embed a YouTube video, use this:
[YOUTUBE ] xxxx [/YOUTUBE ] (without the spaces)

the xxxx is the end of the YouTube link:

http://www.youtube.com/watch?v=cMN32gfl50Y


So, the link above would be: cMN32gfl50Y[/YOUTUBE ]
Without the spaces, of course. Result:

[youtube]cMN32gfl50Y