yeah i did and theres nothing there
If the pre-order was paid for during the PSN downtime, there won't be a a DLC code on the receipt, as on couldn't have been generated in that timeframe.
Since you say Doom, that would mean you pre-ordered from Gamestop/EB Games? If so, call the store you bought it from, and they should be able to get you a code. I called them today to see what was up with the codes, and they told me to come in, receipt in hand, and they would hook me up.