You have to have the streetpass option for ACNL turned on (I can't remember how? According to an AC Wiki:
StreetPass must be activated on the console in order for it to receive and transmit house data. This is done by following these steps:
Switching on the wireless features by turning the switch on the right-hand side of the console onto the "on" position. A yellow light will appear above the switch.
Open Animal Crossing: New Leaf and go to the Happy Home Showcase. Digby will register the game with StreetPass when spoken to by the player.
Have the 3DS in stand-by mode or switched on when outside in busy areas. Not everyone will have New Leaf or their console activated, but if they do the 3DS may download the data for Showcase.) Then, you have to have your DS turned on. When you go somewhere where someone else has a DS turned on, the streetpass option turned on, and they have ACNL, you will find their character in the HHA Showcase (through the gates where Digby stands.) If you streetpass the same person multiple times, they will give you items like tweeters, balloons, and ice cream. Sorry if that's not that helpful, I'm bad at explaining things :c.