By default I set the install location to "c:\program files (x86)\Rockstar Games\GTA San Andreas"

If your GTA SA folder is NOT here, then please open MEd.ini and change 

C:\Program Files (x86)\Rockstar Games\GTA San Andreas

To the path where your GTA SA folder is.

Example, for me, this would be

B:/Rockstar Games/steamapps/common/Grand Theft Auto San Andreas

Hope you understand that.

~ TheNathanNS