Here's the info on Modbus in LabVIEW from the User Group Last night:

Modbus Library: http://www.ni.com/example/29756/en/ Hardest to use.
NI Labs Modbus API: https://decibel.ni.com/content/docs/DOC-30140
LabVIEW Modbus API:Installs with LabVIEW Real-Time or DSC Modules (overview: http://www.ni.com/white-paper/9337/en/#toc3, don't forget the example "Modbus Library.lvproj" in the example finder)
LabVIEW DSC Modbus IO Servers: Installs with LabVIEW DSC Module (demo here: http://www.ni.com/tutorial/13911/en/). Easiest to use.
Modbus Reference Guide: http://www.ni.com/white-paper/52204/en/