Most likely because its configured for FS9 aircraft that are FS9 work for FSX but the "exits" in the aircraft.cfg will be wrongly configured in FSX.
In FS9 it will look like this.
[exits]
number_of_exits = 4
exit_rate.0 = 0.10 ;Percent per second
exit_rate.1 = 0.09 ;Percent per second
exit_rate.2 = 0.10 ;Percent per second
exit_rate.3 = 0.09 ;Percent per second
In FSX it will look something like this (But don't copy it in to your cfg this is for the 747-400)
[exits]
number_of_exits =3
exit.0 = 0.4, -30.30, -9.0, 0.88,0 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
exit.1 = 0.4, -146.0, 8.0, -7.5, 1 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
exit.2 = 0.4, -42.30, 9.0, -7, 2 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
If you switch to a default aircraft (or a native addon aircraft) it should work.
