If you have copied your FSX to another drive and run it from there, your FSX registry may still point to the original.
When you install Ideal Flight; IF finds the installation detailed in the registry.
You can edit the registry as follows:
64 bit Windows
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\microsoft games\flight simulator\10.0
SetupPath=C:\Program Files (x86)\Microsoft Games\Microsoft Flight Simulator X\
32 bit Windows
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\microsoft games\flight simulator\10.0
SetupPath=C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\
You may need to change the SetupPath value.
SetupPath=M:\Microsoft Flight Simulator X\
Then Install ideal Flight, and that will be the working directory for FSX.
Once IF is installed, a note of the directory is kept in Config.ini, and used from there after. The FSX setting can be changed to anything else if necessary.