Looking at the diagram on page 9 of the datasheet shows the "pulse" connections as the grey and green wires. (not the red and black)
Grey is "pulse output" whereas Green= "pulse return"
Then on page 21 we have a rather useful looking table that tells us that the "pulse output" is "Opto-isolated MOSFET volt free normally open contact"
So what we have is a pair of wires that are connected to the mosfet side of an opto issolator. It will not have any signal on it unless your circuit provides the power.
What I think you need to do is provide 5v from your arduino to the silver wire via a a suitable resistor (say 4.7K) and connect the green wire to your arduino GND. You can then take your reading from the silver wire (at it's junction with the resistor)
