1. On your computer make a new Windows Live account with Costa Rica as your region (an
Outlook address is easiest).
2. On your Xbox make a new profile, again with Costa Rica as your region.
3. Use the Outlook address when you have to enter an email.
4. Go to the tab that lets you press Y to search.
5. Search for Injustice. The first thing on the list should be the full game. Click download and make sure the file size is 7.8 gigs or thereabouts.
6. On your computer go to the Xbox
account migration page. Sign in with the email address you created in step 1 and change your region to United States. (I'm not sure if you should change it to your true region if you don't live in the US.)
7. Signing in to the account migration page probably booted you out of your Xbox profile, so go back to your Xbox and sign in again. You should see the US storefront and the game should still be in your download queue.