just connect your console to live and play your movie. a message will appear telling you that you need a media update and you will have the option of downloading it. once downloaded you will be able to play the video, even when you're offline.
dunno, i only have dialup at home, so i can't comment on that yet...
...although i might give my cell's 3g a try if all else fails. i've downloaded the xbox update today, and will copy it to a disc and see if the manual-update will work