Why can't I change my minecraft skin?

I bought the game card today from Best Buy and whenever I try to change my skin it says that I need to 'purchase the game' first. I don't get it.

Added (1). Edit: I followed the instructions on the back of the card.

Have you activated your account on mojang yet?

It takes a few days to process once you enter it. You probably can't play the game yet either.

I purchased minecraft 1 year ago and was fully able to play it. Now I try to change my skin and it says I have you purchase minecraft