To add a youtube clip or any streaming media, click the button.
 
You will see [media][/media] appear in your post.  
 
Add a url to this so it reads [media]http://your.website.address.here[/media] and your video will be included to your post.
 
Here's an example.
 
[media]http://www.youtube.com/watch?v=IH-l3XI3--k[/media]