Well you know how you have the card (forgot what is called) and you use it to get to other towns, your friend code is post to be on the bottom left. I know your post to use your 3D DS code but can you put that on the card?
I believe what you are talking about is the Dream Address Code. The code doesn't get put on your card until after you have unlocked the Dream Mansion and built it. The only other code that I know of that lets you go to other towns is the 3DS code which both you and the other person have to enter before visiting each other. If that is not what you are talking about I apologize but I am unsure what you are referring to.
Intelligence plus character, that is the goal of true education ~MLK