+5 internet points for spelling "gypped" correctly.
I don't own the game or anything but my guess is that it was only in the first printing of the game. Amazon probably never took down the description.
Two options:
Contact Amazon, you'd probably get a partial refund or something maybe.
Contact whomever publishes the game. They might be able to help. Though if it was a first run type deal, they'd probably tell you to take it up with Amazon.
EDIT: Searched on the googles a second and someone mentioned the same thing but with the 360 version. They emailed WB and got an auto email back mentioning missing DLC. He never did update his thread on gamespot though, so I don't know if he got the code. Probably your best bet.