So I bought the game today and after playing some of the story mode, I kinda wanted to check out the online stuff.
However, when I tried to redeem the code that i got in the box, it wouldn't download it. It started to but then it just stopped and a message popped up saying something like "Cannot download Mortal Kombat Online Pass".
 
Now when I try to redeem it, it just says that it's an invalid code! Anyone else that has had this issue or something similar?  
I just want to get online and get murdered!
 
Edit: When i try the code at xbox.com it says that it's already been used... but i wasn't able to download the thing.... Halp!

I know you said it didn't let you download it but I would suggest checking your  Xbox download history.   It quite could have possibly redeemed the code but failed to download the first time because of the marketplace crash earlier today. 
 
Id suggest checking on your downloads.   https://live.xbox.com/en-US/Download/History     Go here and see if your  kombat pass doesn't appear on the list.  If it does try downloading it.