I'm about to go back in and see if it kept my bonus content.
But you need to enter the shorter of the 2 codes Steam give you.
In multiplayer...click on "gamesas" it will close the game and take you to this website. then enter your code and click "redeem". then go back into the game...
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i7 950 OC'd @ 4.15Ghz
12Gb Corsair Dominator UC'd to 1402MHz
2-Way Sli GTX 470 (synced)
OC the twin 470 cards to:
Processor Clock - 1510 (!)
Memory Clock - 1820 (!)
30in HP Monitor @ (2560x1600)