If you get a youtube video it'll look like this as a link
http://www.youtube.com/watch?v=xVUSp1V3cVw
Use the YouTube BB codes to embed it into your post and place the code AFTER the "v=" in the URL into the edit field
eg
[youtube ]xVUSp1V3cVw[/youtube ]
(I added a space after youtube so you can see the code here
Then it'll embed like so
HOWTO: Embed video on the forums
Moderators: Board of Directors, Command