Step 1)
Click on "Insert Media" on the Add YaBBC tags panel (5th from Left)
and it will add a code as shown:
Code:

[media][/media] 

 
Step 2)
Go to the YouTube video that you want to add.
And look under URL and copy that code and paste in between of the Media tags as shown:
Code:

[media width=500]http://www.youtube.com/watch?v=C_Z-ptJ19zo[/media] 

 
Note: The width=500 can be added to specify the width