You can redeem the code through Steam's product activation section:
1. Open Steam
2. Click the Games drop-down tab located near the upper left corner and select Activate a Product on Steam...
3. A new Steam window will appear. Read through Steam Subscriber Agreement and click I Agree if you comply with the terms.
4. Enter the code and click next. Steam will verify the code. You will receive a message stating if the code is correct, incorrect, or already been used.
If Steam accepts the code you will be able to download the four DLC map packs through the Black Ops II in-game store free of charge!
TopFighter | Community Forum Moderator
Modern Warfare, Black Ops, Ghosts