Via FTP: dev_hdd0-game-LAUN12345-GAMEZ-your game (in this case, mw3)
Via Removable Storage Media (USB flash drive, external HDD, etc): GAMEZ (create folder if not already there)- your game
Edit:It might be GAMES instead of GAMEZ (I forget)
Also, if your game doesn't have the right title ID, use a PARAM .SFO editor (Google it xD)