Step 1 : Download and install GSC Studio from here :
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Step 2: Get the mod menu that you want to use and download it...most are .rar or .zip,so extract it on desktop.
Step 3: If you have a multiplayer menu create these folders on your desktop (maps>mp>gametypes)
Step 4: Get the menu and drag it into the gametypes folder and if its not named "_clientids.gsc" rename it to that without the quotes.
Step 5: Load bo2 and go to to mutiplayer and go custom games but don't start it.
Step 6: open GSC STUDIO and push connect>CCAPI and it will bring a box with your ps3 and click connect.
Step 7: Then push on the editor tab and "Inject Compiled Script File" and look for the maps folder on your computer you just made and push it until it gets highlighted and click open.
Step 8: You should see your gsc in box and click inject wait for it to say "Injecting Done" then you can start
Hope i helped