That indicates your FSX registry is missing or corrupter or pointing to the wrong folder.
If you made mistakes like moving the sim after installing it, you must reinstall it. FSX won't work by just moving its folder. Also, if you installed the FSX Steam version together with FSX and then uninstalled the FSX version, it will cause issues to the registry too, so you'll have to uninstall the Steam version and reinstall it with not FSX installed.
Also, FSX must be updated to SP2 or Acceleration. If your FSX is RTM or SP1, none of our software will work
This are just some of the most common case of the installer "not finding" FSX.
