You needed to vote in the injustice battle arena weekly contest on Facebook. You could chose what system and they also gave out Xbox flash avatar shirts or Injustice ps3 theme. It was sent to the first 5,000 voters if I remember. I wouldn't waste $$$ on the eBay scalpers selling it for $20 or more. Show is awesome death stroke is in it as well.
It's nearly impossible to get one now, not unless you were top 5,000. there's those other methods, but they're not working for me anymore.
Oh and if you were top 5,000 and never received your code, you can go to there support site at "http://support.wbgames.com/ics/support/ticketnewwizard.asp?style=classic" and input your information and get one.