On the home screen, look at the very top bar above all of the apps. Then, click on the orange square smiley face. Once you're in there, you should see your mii & 3DS FC. Then, go to the top right corner, click the button that has the plus on it, then click internet. Then, just enter the person you want to add's code! You'll have to tell then you added them, though, because they won't get a notification.