The default setting is this (see image). It shows 3 frames before and 3 after. Shift Space toggles the backlight (onionskin) on/off.
The functionality is very straight forward – so I’m thinking there’s something else going on. Are you trying to onionskin imported frames? They are reference frames and will not show up on the backlight. Once you draw something within AP, everything should work as expected.
Please let me know if that’s indeed what is happening (or not).