Sometimes Youtube links are more complex, like when you select a video from a channel. If this happens, copy everything between "v=" and "&"
(http://www.youtube.com/watch?v=YtiGxKCESos&feature=channel_page)
Now, press the "Insert: Youtube" button which is at the bottom of the list of codes that are below the smiles.

A box will come up. Now, paste the code you copied earlier into the box, and press "OK".

The following code should appear in your text box:
CODE
[youtube]*Your Youtube Code Here*[/youtube]
For me, it looks like this:
CODE
[youtube]Chm5bzCox04[/youtube]
And now your video should appear on the forums when you post it: