It's possible *another* addon has corrupted your FSX.CFG folder, removing a whole Simobject paths ? Check your FSX.CFG at %APPDATA%\Microsoft\FSX, 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's a mistake caused by another addon product, because it's default with any FSX installation.