First: I have not thouroly read the TOU so this thread may not be allowed (something to do with pirated software, dlc)
Second: *remove if not allowed* you need two things, to get dlc and activate it; search your game code (my mw3 is BLUS3038.) on google followed by unlocker *e.g- BLUS3038 unlocker* and you will have files to download, each downloadable dlc .pkg and a .pkg intended to install after the dlc to enable them. (Commonly known as a dlc fix)
If you do not possess this you are running a HIGH RISK of:
1. It not working
2. Getting banned from the server (possible console ban) for trying to run pirated dlc.
So look THOUROLY on youtube on how to do this.