of course you can. Just click the you tube link button on top of your posting box and between the square brackets with the words you tube in, drop the number of the you yube video. Its the last part of the web address when you are on the you tube page.
so for example, the following you tube video
http://www.youtube.com/watch?v=xb9w3lO0Q9E
has the code
xb9w3lO0Q9E
which you drop in between the you tube brackets something like this
...YOUTUBE]
xb9w3lO0Q9E[/YOUTUBE....
and it should end up looking like