You put the part of the YouTube URL after the "v=" bit and before any "&feature..." tags (assuming there are any) between youtube tags
Like so.
{youtube}mAgDKR0L2RA{/youtube} -- only you use [ and ], not { and }
Actually if you just select the "mAgDKR0L2RA" bit and then hit the "add Youtube Tags to selection thing while you're creating a post it'll add the tags for you.