Ensure the DLC is in Hdd1:\Content\0000000000000000\TitleID\00000002 .
Even if the files are in the right place, the DLC may appear with a "locked" icon or fail to load if it isn't properly licensed or updated. Unlocking with XM360 xbox 360 aurora install dlc
Double-check that you didn't accidentally put it in 00007000 (God/Games folder) or 000D0000 (XBLA folder). It must be in 00000002 . xbox 360 aurora install dlc