Geist needs to be 'in focus' to scroll up/down with the arrow keys.
In Ableton, click on the box that surrounds Geist. The box is now highlighted and 'in focus', allowing for keyboard input.
In Studio One v2, it doesn't work as well. Keyboard strokes are sent both to Geist AND to Studio One. Using the down arrow will select the next item in the Geist browser, but will also select the next track in Studio One.