The output of this motion detector is a standard Form C relay, with common, normally open and normally closed terminals. The 28VDC spec on the output is just the maximum voltage and current that the relay contacts can handle.
You'll need a 4-wire cable to connect up to this, using 2 wires for power and 2 for the zone input to the M1.
Wire up the power terminals on the Optex to the VAUX power terminals on the M1 (or to a 12VDC aux power supply if you've reached the limit of the power the M1 can provide).
Then wire up the NC and COM terminals on the Optex to a zone input on the M1 using the other two wires. If you are using EOL resistors on your zones, use them here. If you are paranoid about someone tampering with the sensor, you can also wire up the tamper terminals with a second EOL resistor and configure the M1 zone for 4-state zone wiring. See page 9 of the M1 manual for how this gets wired (Figure 1).