Menu Button is somewhat similar to Chooser: Set the <Options> element to the menu options you would like displayed. When an option is selected the <Value> element would be set and <On Select> fired.
The <On Select> event handler would naturally start with a check of the <Value> element (using an ancestor reference) which will usually flow to a branch with exits matching the different options, flowing to actions handling each option.
Menu Bar and Menu Item are part of an incomplete implementation of menu display support (which would also include a Menu element, yet to be implemented). In the meantime, you can use them to display items inside a bar if you wish.
For best results, use the Firefox browser..