Hi rezar25,
The Modbus library can be downloaded at the link in RavensFan's post: http://sine.ni.com/nips/cds/view/p/lang/en/nid/201711
The Modbus library is an older product and not fully supported anymore. As the note on the download page states, the DSC (Data logging and supervisory control module) is reccommended:
"Please note that this is an example program and not an official NI Product. For a more complete, fully supported MODBUS solution, NI recommends the LabVIEW Datalogging and Supervisory Control Module"