Hello Claire,
Make sure you have the belt on the motor first in the middle of the pulley in the grooves then pop it over the wheel and turn it so the belt finds its own grooves, turn the wheel by hand and look at the belt to see what it is doing, if the belt comes off from the wheel then the wheel may be slight out of alignment.
DO NOT GO INSIDE THE MACHINE UNLESS THE PLUG IS OUT OF THE WALL.
Ken
drivewithken@blueyonder.co.uk, July 2007