It is a mosfet mod kit i bought online, this is the wiring diagram.
Although not my site, Nonameboxmods, I am the designer of that board. It has a 3 position switch: 1/Left-Off fire switch and voltmeter are both disconnected, 2/Middle-Fire switch connect and voltmeter disconnect, 3/Right-Fire switch connect and voltmeter connect.
For the voltmeter to function, you must have the slide switch in the third position (right) and the fire switch must be pressed. It should function with or without an atomizer attached. If it is not functioning as stated, there could possibly be an issue with the slide switch. If you suspect so, you should contact Tom of Nonameboxmods and ask for a replacement.
Also like EverPresentNoob noob mentioned, you could alternately wire the voltmeter to the 510+ and 510- and you may have better results.