Lookout is the MODBUS master, so the M-series PLC is the slave. You should not use any of the READMODBUS, WRITEMODBUS, READMB2 or WRITEMB2 command since this will make the PLC the master and therefore conflict with the Lookout role.
As a modbus slave you don't have to do anything other than ensuring that Lookout and the PLC both talk on the same communcation baud rate and settings (default on the PLC is 38,400,8,n,1). The PLC automatically recognizes the modbus protocol and respond accordingly and even if the PLC has a blank program this still work.
If the serial port is to be a dedicated modbus port, you may consider executing the SETPROTOCOL command to fix it as a modbus only port. This can reduce CPU overhead in determining the correct protocol and can reduce errors too.