Too add friends on a 3DS. Go to the main 3DS menu. You will see a set of icons at the top, one is a pencil, next to that one is a orange smiley face, click on that, u will be lead to a friend's list. You will find yours on your mii card, to add a friend u have to have internet connection on your 3DS. So click on "Register Friend" and then Internet, enter their code and you can enter any name after that. Once they have yours, your connected with them.

Thanks, that was really helpfu^^. I had been looking around the mii plaza and creator trying to find that.

You're welcome. Glad I could help. hehe silly. you won't find them by searching them. ^^

Also if you do trades with people you pass by using Streetpass, I think they'll show up under Acquaintances.