Autoupdate plugin location
From 360Works Product Documentation Wiki
Windows XP (maybe hidden):
C:\Documents and Settings\<username>\Local Settings\Application Data\FileMaker\Extensions\
Windows Vista, Windows 7, and Windows 8 (maybe hidden):
C:\Users\<username>\AppData\Local\FileMaker\Extensions C:\Users\<username>\AppData\Local\FileMaker\FileMaker Pro Advanced\12.0\Extensions
And on Mac OS X:
/Users/<username>/Application Support/FileMaker/Extensions/