In the media URL bit you paste the URL from the Youtube video example:
http://www.youtube.com/watch?v=Hi4ccVBsZEM
Then press "retrieve information".
This will get the video, It's title, the tags and the video description from Youtube and put it on the forum.
The next page will just be all that info in the page and you can either scroll down to the very bottom and press "Submit Media" or edit any of the descriptions, tags and titles first then press Submit Media button.