Make sure that the fuel tank selector on your default aircraft (the aircraft that appears on the FSX Start Screen when you first start FSX) is cet to ALL. This is one reason why the C172 is a good choice for the default aircraft in FSX and switching from it to the more advanced addon.

If you want, you could (maybe should) get the Safe Startup flight for Complex Aircraft from Flight 1: http://www.flight1software.com/files/FSXSafeStartup.exe

Note: You can re-save the flight in a new location, with different weather and start up times, but do not change anything else when saving this flight.
Important! You MUST start FSX with this flight for it to be effective. If you start FSX with another flight, load this one, then go to your complex aircraft, you may get undesirable results.