Do you mean in-game? Go to your 3DS and click the little square face at the top of your lower screen in the Home menu on your 3DS. Then you can register friends with their friend codes. If you mean in New Leaf specifically, then go to the guard up in the train station and ask to open your gates or travel out. If you mean setting it up in ACC, then...
I need to make a sticky on this, since you're not the only one who's having problems setting it up. Here's the short version, though. n.n
Put your AC:NL code in as your 3DS code: 1. Click "My Friend Codes" on the site of your ACC page. 2. Click "My WiFi Games and Settings" 3. Scroll down to "Nindento 3DS Code" and enter your Animal Crossing: New Leaf code there. 4. Congratulations! Now you can click the "Wi-Fi" button for other users and exchange Friend Codes!
Why? AC:NL doesn't actually have a separate friend code for its own game. AC:NL just rips your 3DS code off and uses that instead. So, in theory, you can remove your cartridge (if you have one and didn't install the game) and change your friend code by putting it in a different 3DS. Animal Crossing Community avoids repetition by simply having you use your 3DS code instead of repeating it multiple times for AC:NL and potential future games. I hope this helped. n.n
If you FCR me, please don't send a blank msg. I don't like "100% strangers"
I'm going to be your friend, right? So why not start a conversation? n.n