Use the Loadmanager witch comes with PMDG package, unfortunally with this program only passenger and cargo can be adjusted, and you must reload the aircraft, after reload use the freeware fuelloader (on downloadpage PMDG), to plan your Fuel for the flight, or adjust your fuel in FS9 PMDG-menu
Or use FSpassenger you can adjust passengers, cargo and fuel.
Very importent:
After changing any off these data, update your FMC Perf-int and Takeoff-Perf data pages
Regards Willem, EHM1328