It's possible your FSX.CFG has been corrupted. The most common reason for this, is that you (by hand-editing) or another defective addon, has corrupted the FSX.CFG file, removing a default section that must be there, in order for the sim to find the scenery objects and many other library objects used by default scenery.
Check your %APPDATA%\Microsoft\FSX.CFG file and and look all the lines containing this:
SimObjectsPath=
Be sure there's a path listed for the Simobject\Misc folder, and all the numbers are in sequence. If there isn't, it'a mistake caused by that installer, our installers doesn't touch that line, because it's default with any FSX installation.