Just click the Youtube tag button (Advanced edit mode) then insert the video's unique part of the URL, which is after the "=". For example, "s7TLjMfOQIE" is what you would insert to display a video from this URL:
http:// www.youtube. com/watch?v=s7TLjMfOQIE.
Just insert those unique characters between the Youtube tags and it will work.