You can print the gear file with wither the chassis or the wheels. I printed my parts on a Makerbot Replicator 2. The settings I used are in the picture.
Assemble the car
Place the chassis on top of the wheels. The wheel with the gear should be on the side of the chassis with the rectangle cut out.
Assemble the littleBits
Plug the battery into the power bit and snap that too the motor bit. Slip the gear onto the motor bit. Snap the bits onto the track so that the gears line up.