I had the same issue. I was able to download the pack for free when I visited the game in the store. You can do this if you press the menu button with Ghosts selected at the home screen and select view in store. There, you should see an option to install the DLC with no additional charge.
We appreciate you posting!
Can you please try navigating to the Multiplayer Menu -> Xbox Live - > Store -> Map Packs -> Onslaught.
Now, when you press A on the map pack you should receive a prompt telling you that you already own the content. You will then see the option appear to install the content.
Try that out and let me know how it works out.