This is usually a result of the aircraft creator adding extra weight to the aircraft's ‘empty weight’ to simulate passengers on board. Obviously when you add more passengers via FsPassengers, you take the aircraft overweight.


Solution:
Find the correct ‘empty weight' of the aircraft. www.airliners.net is a good source for this.
Open the aircraft.cfg file in Notepad. See Google about aircraft.cfg editing in FS9

Under the heading [weight_and_balance] adjust the empty weight to the correct value.
Save updated file.


NOTE:
On some aircraft, carrying 100% fuel, 100% pax and cargo is not possible in the real world. In this situation you would have to decide between carrying less fuel, or leaving some pax behind.