Youtube seems knackered at the moment but to put a video in a post you would take the code of the video, which is the bit after v= in the youtube address and then put it between youtube tags on here. The youtube tags are: [noparse][yt] [/yt] [/noparse].
So for instance, if you wanted to put this video in to a post: http://www.youtube.com/watch?v=u7edztkuz5o

You would take the u7edztkuz5o part and put it between the [yt] tags.