Think in terms of the net acceleration. g is an acceleration. The elevator is providing another acceleration acting along the same axis as g. If the elevator is accelerating upwards, then it adds to g (inertial forces generated by its action will act downwards). If the elevator is accelerating downwards, it subtracts from g. The result is a new net acceleration, call it g', that will act on an object contacting the elevator floor.
For your example, and taking g to be 10 m/s^{2}, if the elevator is accelerating upwards at 2m/s^{2}, then the 'weight' of the man on the elevator floor is
(10 + 2)m/s^{2} * 100kg = 1200N
If the elevator is accelerating downwards,
(10  2)m/s^{2} * 100kg = 800N
