When you add scenery to FSX, the FSX scenery database is updated by FSX next time it is started. Addon aircraft just appear as more aircraft in FSX.
This is essentially how Ideal Flight does it.
Scenery:
Before adding scenery, remember to close Ideal Flight and FSX. Once the scenery has been installed, run up FSX so that the scenery is properly configured by FSX.
After that, run IF and choose to build the indexes: popup menu, IF start page.
Aircraft:
Close Ideal Flight before adding any aircraft. When you next run IF the aircraft will be read in and a set of Aircraft Flight Parameters are allocated for it.
You should always check and adjust the Aircraft Flight Parameters to suit the aircraft.
In the simulation Addon Menu, use the IF Fuel Report to find out the correct range for your aircraft.
Steve