Well is it 56 mV or 5.6 Volts? Hmmm....Depends on what range you have set on your meter.....
Ok, looking at the front of the regulator ( where the numbers are...) the connections are; 1,2 &3 left to right.....connect your meter Black lead to the tab, now connect the Red lead to pin1.... What reading do you get? Set the meter to the 20 range DC volts....then transfer the Red lead to pin3, what reading is on the meter?
Apologies if this is all simplistic, but I dont know what electronic knowledge you have....( I'm a retired engineer,with an electrical/electronic background)