In your map at least one idea is always selected. This is indicated by the outline encircling it.
By default, the focus (left-most idea) is selected. Furthermore, no matter which idea is selected, if you click on the background, the focus will be selected.
The ARROW keys browse the map. For example, when you tap the UP ARROW, the selection circle will encircle the idea immediately above. However, if your selected idea is already at the tip, the selection will wrap (move to the bottom). Tapping the DOWN ARROW will wrap at hte bottom.
If you tap the RIGHT ARROW, the subordinate ideas will automatically be revealed. This auto-reveal also happens when you tap the LEFT ARROW when the focus is selected.
If you hold the COMMAND key down while using any of these ARROW keys, the selection will shift to the extreme in the arrow’s direction.