There are no problems having the sim installed in any drive or folder, provided the registry keys of the sim are pointing to it, which is where the installer looks for to find the installed sim.
If you are SURE your supported sim is installed, but the icons are greyed, it’s likely you have a problem with your sim registry keys, which are wrong, missing or pointing to the wrong folder.
With FSX Steam, it might happen if you had FSX (regular version) installed before, installed FSX:SE, then Uninstalled FSX.
If you installed FSX:SE from Steam, but never launched it, so the registry keys are not there. They are created by the Steam interface the first time you launch it from *there*. You can launch FSX:SE from its own icon under normal use, but you must launch it at least once from the Steam program, to have the correct registry keys created.
