Your Optidriver has a serial RS-485 port that can be talked to with Modbus RTU. Your myRIO-1900 does have UART ports but only in 3.3V LVTTL level. So you need an external 3.3V LVTTL to RS-485 converter too. Something like this would work: https://www.hicomponent.com/5V-3.3V-ttl-to-rs485-converter-uart-to-rs485-converter.html
You might also be able to use an FTDI RS-485 to USB cable but if you never have played with Linux before, which is the OS running on the myRIO-1900, it might be too much hassle to get the USB adapter properly recognized. It is not always a plug and play experience but sometimes a plug and pray one.
After that you need one of the several available LabVIEW Modbus libraries to start to try to talk to the Optidrive after you configured it through its display to communicate with Modbus RTU.