Make sure you are on the latest Title Update (TU) for the games to ensure the DLC loads correctly and to avoid any "Corrupt Content" errors. Download Link: [INSERT YOUR LINK HERE]
Use XM360 to ensure the DLC is showing as "Uncut/Unlocked" so the game recognizes them immediately. Borderlands [Jtag/RGH DLC]
Navigate to your HDD or External USB: Content/0000000000000000/ Make sure you are on the latest Title