Do you have the NI Modbus library installed? You can download it here:
http://www.ni.com/example/29756/en/
Put the contents of the “86” folder into your Program Files/National Instruments/LabVIEW [version]/ folder
If you are using an RMC75E, I would strongly recommend using the dcsRMC75E version of the driver as it is completely dependency free.