If you have vB4 you should already have that capability..!?
When making a post look for this button..
It is for embedding videos. If you are asking how to do it.. all you need to do is take the url of the video (from the address bar "http://www.youtube.com/blahblahblah/v/whateverelse ) & put that in the box. Works just like an image.
IF you dont have vB4, then this thread is pointless & in the wrong place, so you need to post somewhere else.

(vB3.x)