1. Go to the YouTube Video you want to link.
I.e:
http://www.youtube.com/watch?v=2zw8S...eresotherstuff
You'll see the video's id.
v=2zw8SmsovJc
The video id is:
2zw8SmsovJc
Put that between the youtube tags. I.e:
[ youtube]2zw8SmsovJc[/youtube]
^ without the space.
Which then displays the video: