The RS-485 wiring has Sg (signal ground) driver lines A and B, and a terminal for TR. I suspect TR is terminating resistance, but it connects to the A driver on one slave device only. Who knows?
Furthermore, that document does not have a Modbus slave table of registers/ addresses and the associated units for the data values. I'd want to find that document before I spent one minute on anything else Modbus.
To connect, you need a Modbus Master, because these are slaves.
It appears that the manufacturer offers a software package. http://www.kael.com.tr/eng/pdfler/MULTIMET-01-PC eng.pdf
You might want to see what that software does and whether it meets your needs before struggling through a Modbus implementation.