Cannot establish serial communication from a sensor
Your sensor has some different modes of communicating:


Modbus, a software protocol that can be run on the electrical standard RS485.
Making manual Modbus frames on the terminal seem improbable.
NMEA, combined software and electrical protocol/standard. Can be run on RS485 and others. Could probably be done on the terminal but it also requires some protocol frame building.
SDI-12, combined software protocol and electrical specification. 5V logic levels. SDI-12 should be fairly easy to use with the terminal but you would need a SDI-12 to RS232 converter. 
Analog current/voltage. Directly measurable by an ADC or voltmeter.


You have to decide which one to use. I would test 4, first to see that the unit is in working order. Attach supply to pin7 &amp; 8. One 1k resistor from pin 2 or 4 to ground. Measure the voltage over the resistor and see if i changes with whatever parameter was set in the factory:


  "The analog measurement outputs are configured at the factory based on customer specification" //From the manual