Or use a battery pack of 4 cells, 6V no regulator required.
PP3 are a pretty useless power supply except for very low current circuits.
Voltage is not the issue here , you need more current.
Phone chargers are 5 V mostly, do you have one of those ?
wex52:
indicated that the Arduino would be sufficient for a one-motor demonstration.
Only for a very low consumption motor.
Most require more current.
BTW
Delay(500);
What is that for ?
EDIT
http://www.maplin.co.uk/p/4-c-battery-box-hf95d
One of these clips to your battery clip and gives 6V which you can use directly.
For safety though you should use current limiting, a wall wart would probably be better