What is the white wire for on this voltmeter?
My guess would be that black is ground, red is power supply (5 - 28 volts) and white is voltage to be measured (0 - 99 volts) - but that is only a guess, and I would examine the unit carefully, comparing it to the schematic provided,  before applying any voltage.

Personally, I would not buy anything that did not come with the necessary documentation.