This forum is a bit old, so you will most likely be better off creating a new forum post with your specific questions. If you're using NI SoftMotion, the Getting Started guide that the previous user linked is going to be a good start: http://www.ni.com/pdf/manuals/375592d.pdf .
If you are not using SoftMotion and just having difficulties with the ModBus communication, it may help to post on the Industrial Communications forum here: https://forums.ni.com/t5/Industrial-Communications/bd-p/260
