Players can play both octaves through the first GUI screen. Shift clicking a note will play its triad.
Clicking on the paper in the top right will take you to the scales menu. Select a scale and you will be brought into hotkey mode, which allows you to quickly change between keys outside of the GUI.
Depending on whether you've selected Quick Play, you can play each note by switching your hotbar slot. If Quick Play is not enabled, you will have to left/right click each note. Right click being one octave higher.