If you are using two USB to RS485 convertors, then you wouldn't need the MAX485 IC unless you wanted to add a third node such as an Arduino onto the bus, in which case you would attach the DI and RO pins to the Arduino's serial GPIO pins. You also need to connect the DE and RE enable pins to a GPIO port to be able to toggle the MAX485 driver between transmit and receive states.
Alternatively, you could use one USB to RS485 convertor connected to the PC and a MAX485 connected to an Arduino and send data from the SSCOM terminal application to the Arduino.
It includes additional logic to automatically enable DE / disable RE on data transmission. It's one of the approaches outlined in Jan's book linked previously.
