If you edit your post and put the video url in it instead of the link code like this
http://www.youtube.com/watch?v=oDVnZwwcTzU
the forum will automatically convert it to this
[youtubevid]oDVnZwwcTzU[/youtubevid] when you save it and this code will embed the video like this
Now you try

just cut and paste this http://www.youtube.com/watch?v=oDVnZwwcTzU into your post.