Some meters have a built-in diode for protection. Generally, the meter will say, "with diode" on the meter case if it has one inside. Some meters will simply have red & black wires or + and - symbols on the case which denote a built-in diode.
The + side of your meter will go to the correct voltage terminal on the power supply depending on the meter's voltage printed on it's case. The minus side of the meter goes to the gameboard's edge connector marked "coin 1" or "coin 2" (usually coin 1 if you only have one meter, coin 2 is for an additional meter if desired to meter coin switch 2).
If the meter has no diode built in, you add one by connecting a 1N4004 diode across the meter leads. Cathode (banded) end of the diode to the power supply positive voltage of whatever voltage the meter is. Negative side (non-banded) of the diode to the gameboard edge connector coin 1 terminal (or coin 2 if this is an additional meter).
Randy Fromm did a nice write-up on connecting meters years ago in Replay magazine.