Because DLC is usually tied to a specific console or profile, you must "patch" or "unlock" it to work on your JTAG/RGH console.
In your library, highlight the game, press (Y) , and navigate to "Title Updates." Download and Enable the latest update (e.g., TU 11 for Black Ops 1).
This version is notoriously difficult on RGH consoles. It often requires specific "Compatibility Packs" found in the 00000002 folder to reach the main menu. Jtag/RGH Tutorials #5 Installing DLC