Basically, all you have to do is click the YouTube button and the shortcut will appear in the reply box. Copy the URL from your address bar of the video you'd like to post and post it in between the codes.
e.g
[youtube.][/youtube] (that's just the code, minus the full stop which I had to add to make it not work and show up)
[youtube.]http://www.youtube.com/watch?v=92IGkLece5E[/youtube] (URL posted in between the brackets.)
Sorry if I sound patronising. Really simple but kinda hard to explain. If you have any trouble give me a shout.