Peter,
By all means download the Plug in and Try it, it is in "Finished" Plugins. Go Up a few post to where you see the "MSI" installer. It willl install everything for you and is a lot easier to deal with than the original manual file install.
I dont have a Mod I/O to test with here, but, there should be no probem. I am Grabbing and sending stuff directly to the Modbus Memory Block in Mach.
The only thing you would have to do, is set up the Modbus Config dialog, then make sure the plug in dialog (the Help files under the Help button will explain it all), match your dialog.
Note: the Plugin will allow upto 30 inputs and 12 outputs. The modbus functions you dont want to use, just put a -1 in them.
Let me know how it goes for you. BTW: I am using a Radio Shack USB to Serial converter and if you are maxing out the Plugin, you have to stagger your scan times on your io and set up higher time out. I will try that FTDI converter you recommended.
scott