Before doing anything, see what you are set at now if anything--it has to be something. Put a voltmeter in the output terminals and plug in the unit. Green light should come on. Turn the pot and see what voltage range it has.
If it acts like an ADJ should you are in business. If not, then let's see what to do with the switch. I will check how that works mean time.
EDIT--that switch has six pins underneath. The centre moving switch part goes to either end. Don't see that moving part in your photo, but it looks like (be careful you match the back with the back and front with the front pins-- I think--) you can run something across between the middle pins and the right hand pins (looking towards the circuit board as though through the hole---the right set is for single stage on mine.