The tension on the belt is adjusted using the three screws/nylon motor mounting bushes which secure the motor to the casing. The belt should be slightly loose - i.e. you should be able to squeeze it slightly with your fingers where it comes away from the small pulley on top of the motor. Be careful not to over-tighten the motor screws as it is possible to strip the threads in the top of the casing.
Most importantly, there should be a white plastic washer which fits tightly on the top of the small motor pulley once the belt is on. This stops the belt riding off and damaging the top cover.