Lets say I wanted to post this video:
https://youtube.com/watch?v=u7qSx...ture=fvwp&NR=1 (using this for an example as the part you need is sandwiched in the url)
You would type this in (without the spaces I added so it would show the code):
[ youtube]u7qSx2WLqm0[ /youtube]
and you would get this:
So... you put in the youtube tags and in between it the url for the video starting after the = sign and stop before the & sign if there is one.