What you have to do is use the youtube code after the “=” sign at the end which will have random numbers and letters.
Click the Youtube button 
and paste that code
Example: If this is the link to the video…
Use this part of it http://www.youtube.com/watch?v= xAuQmJzt_q0 in the button.
Ergo…
xAuQmJzt_q0
Pasted into
[yuotube=425,350]xAuQmJzt_q0[/yuotube]
And viola!
[youtube=425,350]xAuQmJzt_q0[/youtube]