That device is a two wire current loop output. One must wire a series resistor and use the voltage drop across the resistor as the signal output to be wired to a digital input. So what source voltage are you going to use? The spec say you can use anything between +4 and +24 volts. I assume you want to be able to wire it to a arduino digital input pin? If so and you don't wish to use any kind of op-amp comparitor circuit, you will have to carefully size the sensing resistor to ensure the the sensors high and low states are converted to legal high and low arduino logic voltage levels. I think around 200 ohms might work, assuming a +5vdc Vcc for the device, but I would want to test it out first.
Figure 7 shows a typical interface using a 100 ohm sensing resistor feeding a comparitor. Figure 4 shows both high side and low side interfacing using a series sensing resistor. Again this device is not as simple to interface to an arduino digital input pin as it might first appear as the sensor is a digital current output and the arduino digital input requires a voltage input, thus the need to convert the logic current signal to a logic voltage signal that meets the AVR legal digital voltage requirements.
So how are you wiring up the sensor to arduino presently?