When posting using the Youtube tag, the bbc needs only the video ID and it will complete the url itself. (Entering any type of url will not work.)
Syntax: [youtube=width,height]videoID[/youtube]
Example: [youtube=425,350]-sDyVAUb8xE[/youtube]
You've got to put the video's ID in between the tags, like in the example above. The id will be everything at the end of the URL address after the
v=[something or other]. It's the [something or other] that you want.
ADDED: Let me make this a little more clear by using an example. I just did a search for "2006 MTV Movie Awards - Gnarls Barkley" on Youtube and got several results, so I'm not sure which one specifically you are trying to post, so I just clicked on one of them.
When the page opened, the URL up in the address bar is:
The video ID is the part of the URL that I have made bold and put in blue.