If you bought the season pass from a retailer rather than through the PlayStation Store*, it should have been printed on your receipt^. It could also be on a card in a spare PS3 game case (some retailers are odd like that, have a couple spare cases lying around as a result of getting PSN credit).
^Sometimes however mistakes are made, and the person at the register can forget to print it.
*If bought through the PlayStation Store, there is no code, and no action other than the "download" is required (the reason it's quoted is because most season passes, much like on-disc DLC, are usually approximately 100KB activation files).
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When I was in GameStop it looked like the Season Pass was a redeem code on a card, separate from the game box and not on a receipt. So if you have that card or whatever was in the little Season Pass envelope, I would think you code would be on that.
As the others said, the code was on the bottom of the receipt. If you lost the receipt, the only thing I can suggest you do is repurchase it through PSN......or wait and buy the DLC on the 13th when it becomes available to purchase without the pass.