Friend codes aren't specific to games anymore, instead, each system has a different friend code. You system's friend code should be able to be found by going to the Home Menu, and pressing the little orange smiley face, which should be near the top. If you have already done whatever you have to do when you first open it, (I don't remember exactly what it is.) you should be able to view friend cards. The one with your Mii on it is your friend card. Your friend code should be on your friend card.