The menu bar is activated by clicking on a menu button with the mouse.
Once it has been activated, you can navigate around the menu system simply by moving the mouse. Moving horizontally in the menu bar activates different menus.
Moving vertically will highlight a menu item.
If a menu item holds a submenu (indicated by the 
icon), the submenu will be opened as the item is highlighted. The focus will remain on the menu system even if you move the mouse away from the menus.
Click a highlighted menu item to execute that action.