I found the option to turn on SpotPass in Animal Crossing: New Leaf, but I haven't found StreetPass yet. Am I just not far enough in the game for the feature?
You need to unlock the Happy Home Academy (which will be inside Nook's Homes) in order to access StreetPass features. This is where people and their houses will appear.
To unlock this, you need to pay off your 10,000 Bell down payment (when you end up with a house instead of a tent) to Tom Nook. Once you get a house, the Happy Home Academy will be open, and this is where you can see the houses and such of people you have met over StreetPass.