May not be able to get account related assistance from SE right now, since it is the weekend.

The code is often included somewhere in your receipt for your purchase--be that one you were able to print at the time of purchase or that got emailed to you afterwards. Depending on who you purchased from, there may also be an interface that you can log into to retrieve your download content and such. Within that framework, there should be a way to list your code. Gamestop and Amazon were doing this last time I purchased DLC from them.

If you can't get at it through such means, you should be able to contact who you purchased it from for retrieval. Make sure to have your receipt available to provide details to verify the purchase and such.