Got a link to the photointerrupter datasheet?
The MOSFET needs a high voltage to turn on, >2,5V up to 4.5V.
Connect one side of the motor to +5, the other side to the D pin, the S pin connects to Gnd.
Connect the Gnd to the Arduino Gnd so they have a common reference.
Connect a diode, like 1N4001, or whatever you have handy. Something that can handle 70mA.
Cathode connects to +5, Anode connects to the same side as the MOSET.
The Arduino connects to the G pin.
Drive the G pin high, the MOSFET will conduct and the motor will turn on.
Drive it low, the MOSFET will turn off, the motor spinning will create some current as it winds down, the diode gives the current a place to go, dissipating itself in the motor.
Add a 10K to 100K resistor from the G pin to Gnd to keep the in low when nothing is connected.
You can use two in series to make an inverter if your photointerrupter normally has a high output.