(original posted: 2012)
When designing a Kivy menu with the kv language I run into troubles with the anchorlayout object.
It seems that you need to put every object inside a achorlayout which itself is part of a anchorlayout.

Sounds a bit vague so an example should do:

AnchorLayout:
    id: topmenu
    anchor_x: 'center'
    anchor_y: 'top'

    BoxLayout:
        orientation: 'horizontal'
        AnchorLayout:
        anchor_x: 'left'
        anchor_y: 'top'
        padding: 8
        Button:
             id: infobutton
             size: (48, 48)
             size_hint: None, None
             background_normal: 'info_button.png'
             background_down: 'info_button.png'
             on_press: root.on_button_clicked("info")

       AnchorLayout:
           anchor_x: 'center'
           anchor_y: 'top'
           padding: 8
       BoxLayout:
           padding: 0
           spacing: 4
           size: (300, 48)
           size_hint: None, None
           orientation: 'horizontal'
           AnchorLayout:
                anchor_x: 'center'
                anchor_y: 'top'
                Button:
                     size: (48, 48)
                     size_hint: None, None
                     background_normal: 'star1.png'
                     on_press: root.on_button_clicked("star1")
          AnchorLayout:
               anchor_x: 'center'
               anchor_y: 'top'
               Button:
                   size: (48, 48)
                   size_hint: None, None
                   background_normal: 'star1.png'
                   on_press: root.on_button_clicked("star2")
          AnchorLayout:
               anchor_x: 'right'
               anchor_y: 'top'
               padding: 8
               Button:
                    id: quitbutton
                    size: (48, 48)
                    size_hint: None, None
                    background_normal: 'quit_button.png'
                    background_down: 'quit_button_ro.png'
                    on_press: root.on_quit_button_clicked()