Hi, I had the same problem. I downloaded map packs for MW3 and I started having problems, I play Hardcore Team Deathmatch and Free For All, and could not find any game. When in the game lobby press the button "Back" and showed me a message (not that means yet) but I play the maps that brings the game by default. Although I can not play the maps I downloaded, I can play the maps that brings the game default. I hope at least you can play these maps. good luck!