If you have pre-ordered the Limited Edition, you just need to update Battlefield 3 through Origin and the Back to Karkand content will be unlocked in your game. It will not show up as a separate file download on your Direct2Drive account.
Here's how to update through EA Origin:
- Open the Origin client on your PC
- Open the Store tab.
- You will now need to go to My Games and update Battlefield 3 to play Back to Karkand.
If you did not preorder but purchased on GameFly:
- Open Origin and click on Redeem Product Codes. Enter your code to redeem the DLC.
For any technical support or key/activation support please see the experts at EA Games Support: http://support.ea.com