Follow the porting instructions (and customize MSPL_UserIF) and you'll do fine -- you have to write the serial I/O routines (remember the 3.5 char turnaround and handle taking/releasing the RS-485 line -- the hardest parts were dealing with the fractional divider for high baudrates and the turnarounds since I did that in software instead of hardware).
And periodically (when you have time.... this takes a while!), you call MSPL_RunModbus. This can take more than 20 ADC samples, so you might need a buffer on the ADC so the Modbus doesn't cause you to miss ADC samples.