I was looking in the forum, and I found this post, http://mylittleforum.net/forum/index.php?id=3942
but it was wrote two years ago, and now is not possible to use this method.

Wich is the best way to insert a youtube video in a post?

I've used the FLASH buttom, but the first option (Video code) shows:
"[flash width= height=][/flash]" (without quotes)

and the second option (Custom flash video) shows a greath blank space like this.

Thanks!