Unlike Black/White it's not in the game, it's on the 3DS. At the main menu, there should be this random square smiley face. (First you need to get a Mii to get your own FC/to access this properly - the Mii program should already by on your system).
Click the smiley, add a FC, make sure they add yours and BAM.
Your FC is now universal with all your games as opposed to separate ones in the past. You can access it (whether it is to get your FC and/or add friends) through the smiley face icon on your home screen.