If the only voltages you get are negative, you could connect the GND (0V) to connector pin 1, and the negative signal to pin 2. After all -50V between conductor 1 and 2 is the same as +50V between conductor 2 and 1.
OP is a bit ambiguous but does state:
TAJS-C:
basic 1/2 second pulses of Negative voltages of more than -12V up to -50v
This what I based my circuit on: converting a 0/12-50V digital signal into a 0/5V digital signal for use by an Arduino. It would work the same for a 3.3V Arduino, by the way.
If you have a signal that goes from -50V all the way to +50V indeed you need a different circuit. A second optocoupler (which goes in place of the diode) could give you two digital outs (one for a negative signal, the other for a positive one), or if you want to be able to measure any voltage in that range you need a voltage divider + offset type circuit to go to a 0-5V analog signal.