app_delegate.rb

class AppDelegate
  def applicationDidFinishLaunching(notification)
    buildStatus(setupMenu)
  end

  def buildStatus(menu)
    @statusBar = NSStatusBar.systemStatusBar
    @item = @statusBar.statusItemWithLength(NSVariableStatusItemLength)
    @item.retain
    @item.setTitle("Menubar App")
    @item.setHighlightMode(true)
    @item.setMenu(menu)
  end

  def setupMenu
    menu = NSMenu.new
    menu.initWithTitle 'Menubar App'

    mi = NSMenuItem.new
    mi.title = 'Hello!'
    mi.action = 'sayHello:'
    menu.addItem mi
   
    mi = NSMenuItem.new
    mi.title = 'Quit'
    mi.action = 'terminate:'
    menu.addItem mi
   
    menu
  end

  def sayHello(sender)
    alert = NSAlert.new
    alert.messageText = 'This is Rubymotion Status Bar Application'
    alert.informativeText = 'Cool, huh?'
    alert.alertStyle = NSInformationalAlertStyle
    alert.addButtonWithTitle("Yeah!")
    response = alert.runModal
  end
end

Rakefile

# -*- coding: utf-8 -*-
$:.unshift("/Library/RubyMotion/lib")
require 'motion/project/template/osx'

Motion::Project::App.setup do |app|
  # Use `rake config' to see complete project settings.
  app.name = 'MenubarApp'
  app.info_plist['NSUIElement'] = 1
end