Where did you order from? I know TD was charging extra for the codes with some cards. I would just contact nvidia or the retailer and they will get one to you. 

There should have been a coupon in the shipping box, loose or taped to the video card box.  Check again then call newegg if it isn't there.

Dear jtrankler,
 
I can recommend checking your Newegg receipt as the borderlands 2 code will be one of the line items if it was available with that product during the time of purchase. If the line item is not on the Newegg receipt then unfortunately this code was not included as part of the purchase. I can recommend contacting Newegg if you should have any additional questions or concerns on this promotion.

It is worth mentioning that the offer says while supplies last