Just an update for those finding this thread w/ Map Pack troubles:
I, too, had difficulty (I bought both Noble & Defiant at the same time), I could not access the Defiant maps for anything - even local custom games. They had the orange star next to them and trying to join the DLC Playlists just guided me to the online store, which informed me that they were already purchased.
I'm here to confirm that if you delete the "BungiePro Confirmation" from your HDD Halo Reach folder, all is fixed instantly.
So much for the thread where it said "this is a temp fix - Microsoft is working on a permanent solution". hahah. That was months ago.
Still working on it, guys? ;)
(nah, I'm just joshing ya - you already got my money! I know I'm not a priority anymore).