Solution
The latest versions of NI Modbus library can be installed through VI Package Manager as described here:
LabVIEW Modbus API.
Older versions can be obtained here:
MODBUS Library for LabVIEW. Find the download link on the right side. Inside the zip file are several versions of the
ni modbus.llb, compatible down to LabVIEW 7.1.
If you prefer having an .
msi installer, there are installers of version 1.1 available for download: