Minecraft prepaid card already redeemed?

So I bought a prepaid minecraft card at 7eleven on July 9th 2014. For the past 2 days the code doesn't work, giving me the same reply after I enter the code: This card has already been redeemed. To download the game go to your account overview page. I really want to play minecraft with ny friends. When I checked the card, the back wasn't scratched off

Guest

Have you downloaded the game yet? Go to your account overview page and download the game for your operating system. Now try logging in with your account. In order to play the game, you have to download it.

Yoshi