With the caveat that this works with the Elk M1 (and the Omnipro may have different power output or need a different kind of zone input or different EOL resistor), this is what I do:
The red wire from the sensor goes to the +12 VDC power from my Altronix power supply unit (the red wire originally went to the Elk's +12 VDC AUX supply, but I moved the sensors off the Elk's limited power capabilities), the black wire from the sensor goes to the Elk's zone negative, and the white wire from the sensor goes to the Elk's zone input. A 2200 Ohm EOL resistor is then put on the panel between the zone negative (black wire) and the zone input (white wire).
Good luck
(I should add that, as I learned from this site, I have bonded the Elk's and the Altronix's DC negatives together)