In the new thread and reply windows, you'll see this button:

Clicking it will add the [ YT ][ /YT ] tags to your message (with no spaces).

To add a particular video to the message, you need to add the YouTube video file name to it, so for instance with this video:
http://www.youtube.com/watch?v=bJAxshN0ARo

you use the YouTube button and add the bJAxshN0ARo bit between them so it looks like this [ YT ]bJAxshN0ARo[ /YT ] with no spaces and what you get is this: