Anybody programming Sketchup extensions should use the Extension class. A good way to program an extension with this class could be: Use a "loader", which should be saved directly to the Plugins / Tools directory. In this file you set up your Sketchup extension. Put your Sketchup extension code to another file in a subfolder of the Plugins / Tools folder. If you would put it diretly to Plugins / Tools, it would be loaded automatically. But with the subfolder and the Extension Ruby class Sketchup can decide whether to load your Sketchup extension. This is needed for extensions, which can be switched on and off via menu "Fenster > Voreinstellungen > Erweiterungen" (should be: "Window > Preferences > Extensions"). Switching off extensions take effect after a Sketchup restart. This way you do not need to edit the file name of the Sketchup extensions as Gaieus mentioned.
Code for the "loader":
require 'sketchup.rb'
require 'extensions.rb'
filename = File.basename(__FILE__)
if(not file_loaded?(filename))
ext = SketchupExtension.new "EXTENSION NAME", "SUBFOLDER/FILE.rb"
ext.description = "DESCRIPTION"
ext.creator = "YOUR NAME"
ext.copyright = "COPYRIGHT"
ext.version = "VERSION"
Sketchup.register_extension ext, true
file_loaded filename
end
azuby