you need the second disk that comes with ODST if you want to play any playlist that has the new maps. I think theyre called the mythic playlists. But you can still play the old playlists with the old maps (social slayer, social big team, etc.) without the newest disk, as long as you have the required map packs for those playlists (Heroic Mapack, Legendary mapack, etc.)
and you cant download the new maps to your xbox as far as I know. you have to have the disk in the machine to play the new playlists.