Seriously, if that's your knowledge of a multimeter and electronics, possibly you shouldn't be digging around inside an empeg.

Current and voltage are entirely different things and if you don't understand that you could potentially break it further.

The voltage will be referenced to ground. The metal case is most likely ground or that screw in the picture where it screws down on the PCB should be ground.

The meter should be on a voltage range (not current).

The reason Mark said use the "right" pin is because that will be the output of the supply. Disconnecting the jumper isolates it so that if there are faults/shorts elsewhere they won't affect things. An isolation test.