On the youtube page you'll see the URL under the video description. For this video the URL is
http://www.youtube.com/watch?v=g4v1rDLIidc
All you need is the part after the equals symbol, though. They all have the equal symbol in the same spot. So you're left with g4v1rDLIidc
Then you just type [youtube]g4v1rDLIidc[/youtube ]
I left a gap before the final ] just to show what to type. Close the gap, and..
• Video Link
VOILA!