Care should be taken with that motor gearbox since it has unefficient motors that are low voltage (up to 4.5V) and high current (over 1.5A each). Higher voltage will melt the collector on the motors! You will need to supply unregulated voltage (make sure your battery pack allow it) to the motors through the Motor shield, solder 0.1uF capacitors on each motor to reduce the electrical interference, twist the wires that go from the Motor shield to the motors. Also, the motor shield is designed for motors that use up to 1A each. I would not recommend this motor gearbox, I had many problems with it. Try to use the Micro metal gearmotors instead.