It's likely your FSX.CFG is missing the line that points to our Simobject folder. The installer will add it automatically, but if you FSX.CFG was already corrupted, the installer won't touch it, risking to corrupt it even more. Or, if you reset the file to default, you should add the line again.
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 folder pointing to the Addon Manager main folder (the one you chose when installing), and all the numbers are in sequence.