It's a key from an Amazon giveaway. You must own Saint's Row the Third to unlock this DLC.
I don't have that fancy game but I tried to use the key anyway and it didn't work for me. I don't know if the key still work or is consider as used.

I will send the key by email.

By the way. I like Defense Grid and I want a sequel. Please support Defense Grid 2 on Kickstarter http://www.kickstarter.com/projects/hiddenpath/defense-grid-2