**DISCLAIMER**I am not responsible for any problems as a result of this advice**
What you do is you put your multimeter Ground lead to the Ground of the power supply. And the Power measuring lead to the power terminal you want to measure. They should all be labelled.
The next thing is the power has to be on. (duh)
Also, you will get readings of Zero if there is no load (a "load" means that there has to be some electronics hooked up, drawing power. In this case, your game board.)
So that's it. If you have already messed with the red knob, then turn it down (H I think == "high", so turn in opposite direction). Test the power. Then turn it up slightly. Test again. Etc, until it reads around 5.0.
Thing is, you probably will have to shut down and power on each time you change the voltage, to see if your game board boots up. Some boards will work right with a +5 as low as 4.7. Whereas some will NOT work until they get as much as 5.2.