The folder…
C:\Users<your_windows_user_name>\AppData\Roaming\SketchUp\SketchUp [n]\SketchUp\Plugins
in which we must save the ruby files, as said in the web first example of SKP, is not accesible for the current user of the system.
It is accessible but it’s a hidden folder. You can make it visible but you shouldn’t need it for most plugins and extensions. Install them by using the extension Warehouse, Sketchucation’s Plugin Store tool and Install Extension under Preferences>Extensions.