the central pin is usually ground, 1 pin will be input voltage level say +9, and one will be output voltage level say +5. the output voltage uses the output pin for v+ and also shares the ground pin. I wont go into detail about which pin is the input without knowing the part number, because they do change sometimes (i got stung by this not long ago, i made a power bus with a 5.5v regulator then a 3.3v regulator in line with it for two different voltage levels. I presumed that because they were the same package and i bought them from the same place the the pins would be identical but they werent).
The $50 robot uses a 5v regulator, you might want to look at that for a working example(just check that the pins on your device are the same)