from Sparkfunwebsite:
"The alarm pin is an open collector meaning you will need a pull up resistor on the alarm pin. "
You can connect this sensor to an Arduino digital pin but need the pull up resistor
If you don't know about pull Up resistors (just an ordinary resistor by the way) read Lady ADA's tutorial on using switches, this is important for your sensor to work correct.
You will also need additional components for controlling the motor. Exactly what you need depends on what type of motor. The thing is that you can't drive motors (except very very small ones) directly from Arduino, because they draw more current than Arduino can supply.
If you can tell us what type of motor you are thinking of (Dc motor / stepper motor / servo motor / ???) then we can give more specific advice
Good luck with your project