Go to every tab on that driver config sheet and check all the settings.
You want the Driver's Station Number to be the same as the master PLC station number (not sure about the ML1000, but I think it's possible to set up some slaves to only respond to messages initiated by the master node number)
On the Polling Lists tab, you want to add the numbers that you think are the node number of this PLC. Most people number them 1, 2, 3, etc, but I had one site that had random crazy node numbers and without the master PLC program to inspect, I would have never guessed what they used. If you don't get the right number in your polling list, the driver will never "see" it. I usually put them in the Priority Stations list.
The port config tab is where you make sure you pick the right windows COM number, baud rate, 1 stop bit, 8 data bits, None for parity. The baud rate should be something that you can figure from the radio modem capability. Most of the old UHF radio modems work at 4800, but we have a couple that use 9600. Most 900Mhz will be 9600. If I remember right, I had to delete the driver and set it up again when I needed to try a different baud.
On the DF1 protocol settings, you want to change the error checking option to CRC. I think I leave the rest of the settings on this tab at the default.
I don't recall changing the settings on the other tabs. I have to delete this driver after each use, to avoid port conflicts with other drivers and I am normally only using it in the process of an upgrade to ethernet or DF1 radio protocol, so I have not kept good records of these settings.
Once you get the driver running, give RWSho some time to populate the results. If the PLC does not appear after 2 minutes, something isn't right and it is certainly possible the PLC serial port is fubar. This is made more likely by the fact it is (was) attached to to a radio modem which is attached to a piece of metal sticking up into the electric sky.