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://forums.ni.com/t5/Reference-Design-Content/LabVIEW-Modbus-API/ta-p/3524019
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: https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000x0QgCAI&l=en-US). Easiest to use.
Modbus Reference Guide: https://www.ni.com/en/shop/labview/introduction-to-modbus-using-labview.html