Launch Property List Editor and open the com.apple.finder.plist file (located in ~/Library/Preferences). From the list of properties, find the one called AppleShowAllFiles.

This is the default application for .plist files, which means that if you double-click a .plist file, it should now automatically launch Property List Editor and open.

Open it and you will see a file named "Info.plist" which you can delete (rename to something like "Info.plist.original") and then drag the Info.plist file which you downloaded into the "Contents ...