If you want t post a video from YOUTUBE you have to put the link of the movie, which you can find in the "embed" field or link on youtube.
In this case the full embed tag is the following and I have bolded the part you need you must copy and paste just that portion of the code without the " "
Being a flash file, you will of course need to use the flash button in the new message window (the button with the little red F) and put the url in there.