You probably have the licenses all screw up mate, if I had to guess. That's what happens when you download stuff on a bunch of different gamertags. So three suggestions.
1. Buy Halo 3: ODST, as others have said. It is by far the best deal.
2. Why not try the Microsoft License Transfer tool. Perhaps at some point your other maps were purchased on another console originally. Log into the account that bought them and move the licenses to your current console.
3. Go into the Forge lobby. See what maps don't show up. IT's also very possible that by downloading the new DLC, you bypassed the Custom Content limit of 151 items. When you have more than that number (of which a mappack takes 3 slots... so every DLC map would take up 13 of your 100 map/film/gametype slots... which is another reason ODST is the better buy), DLC won't appear as an option. So try deleting at least 3 maps/gametypes/films you have saved to your Xbox on that account (plus any number you have downloaded since downloading the DLC, if any). Then see what happens.