In order to read a voltage, you have to have voltage present, so voltage readings are done with the power to the game on.
When you read in your manual or on the pinwiki:
You'll normally be given a hint about what you are measuring.
In this case, TP3 should have 12VDC on it. So we are looking at the DC Voltage, so switch to the VDC section.
Then turn the dial to the lowest number that is above the voltage you are trying to read. If you want to read 12 volts, set the meter on 20.
If you wanted to read 70VDC, you would turn the dial to 200, because that's the next lowest number that is above the voltage you are trying to read.
The next thing to do is to connect your black meter wire to ground.
Ground in pinballs can be the little peg in the middle of the driver board marked GND, or because lots of things are connected to ground, you can put your black meter probe on one of the screws that hold the boards, or you can find a piece of 'ground strap' that looks like a braided ribbon of silver wire. Some of that is in the backbox, on the bottom of the box area.
So you connect your black probe to ground, and then you touch your red probe to the Test Point. In this case TP3.
This is supposed to be 12V, but it frequently measures 11.9V. That's OK.
Let us know what you find.