fake_extension.rb

# Code adapted from https://forums.sketchup.com/t/mac-extension-doesnt-load-when-installed-in-several-locations-user-folder-machine-folder/118206/4

module FakeCompany
  module FakeExtension

    unless defined?(EXTENSION)
      EXTENSION = SketchupExtension.new(
        'FakeExtension',
        'fake_extension/main'
      ).instance_eval {
        self.version = '1.2.3'
        self.creator = 'Fake Company'
        self.copyright = '(c) ' << Time.now.year
        self.description = 'Fake description.'
        Sketchup.register_extension(self,true)
        self
      }
    end

  end
end

main.rb

# Code adapted from https://github.com/SketchUp/sketchup-ruby-api-tutorials/tree/master/tutorials/01_hello_cube

require 'sketchup.rb'

module FakeCompany
  module FakeExtension

    def self.create_cube
      model = Sketchup.active_model
      model.start_operation('Create Cube', true)

      group = model.active_entities.add_group
      entities = group.entities

      points = [
        Geom::Point3d.new(0,   0,   0),
        Geom::Point3d.new(1.m, 0,   0),
        Geom::Point3d.new(1.m, 1.m, 0),
        Geom::Point3d.new(0,   1.m, 0)
      ]

      face = entities.add_face(points)
      face.pushpull(-1.m)

      model.commit_operation
    end

    unless file_loaded?(__FILE__)

      menu = UI.menu('Plugins')

      menu.add_item('Create Cube Example') {
        self.create_cube
      }

      file_loaded(__FILE__)
    end

  end
end