Got It All Achievement and DLC
So the DLC achievements are finally added to the xbox dashboard but they don't unlock. Never mind, it's not all bad news! You can use the DLC to help with other achievements in the main game: specifically the "Got It All" achievement.
This achievement requires you to unlock all the other 48 achievements. However with the DLC there are now 59 achievements, and for this one, you can use any 48 of these 59 achievements to get it. Although the DLC achievements don't register on the dashboard, they do register in the in-game Achievement page, so you can use this to keep track of how many you have.
Once you have 48 shown as unlocked on the In-game achievement page you should get the "Got It All" achievement.