You definitely do not need MSG instructions to communicate with your MicroLogix from a SCADA system.
The configuration of the PLX router looks correct, and it does appear in RSLinx as an Ethernet device. It's the DF1 port and connected MicroLogix that do not appear.
What sort of cable are you using between the PLX router and the MicroLogix ? Does the COMM LED on the MicroLogix ever flicker when you browse the PLX router in RSLinx, suggesting that packets are moving between the PLX and the MicroLogix ?
You might also try configuring the MicroLogix Channel 0 serial port for Source ID =1 instead of 8. While most DF1 Full Duplex Point-to-Point implementations ignore the Source and Destination station numbers, it might be that the PLX router expects the MicroLogix to have DF1 station address = 1.