This worked for me, but it might not work 100 percent of the time.
Me and my friend figured this out thursday night, all you need to do is find a friend who already bought the map pack on his/her xbox then go to your xbox and recover their gamertag on it (if you dont know what that means, its when you basically sign in on their account on your xbox, theres a recover gamertag button that you can see in the main menu when nobody is logged in) then go to the game marketplace and find the map pack download and there should be a little check mark next to it which means you already downloaded it (on their account) so just hit download again and it downloads onto your xbox.
It worked for me, meaning i can play it on my account after downloading it on his to my xbox - but the outcome may not be the same for the rest of u.
-Recover their gamertag on your xbox and log into their gamertag
-Find the map pack download in the game marketplace
-Click download again (free)
-Log onto yours and pray that it worked.