Many older design chargers wont do automatic storage charging/discharging, and it looks like yours is one of those.
The precise storage voltage is not critical, as long as they are not fully charged or fully discharged. Chargers that do storage mode usually aim for 3.85v per cell, but really anything in the range 3.7-4.0v should be OK. Providing you don't over-discharge when flying then it's OK to just store them as they come off the plane.
If you need to discharge down to storage voltage then you could use the discharge feature but use one of those voltage alarms on the balance port to give and alarm when the voltage hit 3.8V on any cell.
You can also minimise degradation during storage by putting the batteries in a sealed container and putting in the fridge.
Be aware that the charger you have has a very low discharge power of only 5W so discharging anything other than very small bateries will take a long time.