Modbus support is installed by default with the DSC module for LabVIEW 8.2 and later. However, this support is an optional feature in prior versions of the DSC Module.
If Modbus does not show up as an option to add in your I/O Server selection window, you can simply repair your installation
of the DSC Module. Be sure to include the option for Modbus support when doing this repair. After this step, you can then add a Modbus I/O Server to your LabVIEW library.
If the issue continues, or if multiple I/O Servers are not listed, it may be that the NI Configuration Manager Service is not running. Refer to this document
for more information on fixing this issue.