Power flows from anode to cathode, just like a LED. So to get a positive voltage out, you ground the anode, then measure the voltage on the cathode.
General convention for low-voltage wiring is that when a colored wire is paired with a colorless (white or black) wire, the colored one is positive.
I just checked my LUX meter with a voltmeter to be sure, and it does follow the convention.
Hook one up backwards, chances are nothing bad will happen; apart from it just not working.