If the communication you need is very simple, like e.g. just to get X modbus register values from the unit then opening a serial og tcp/ip link and sending a fixed string to the unit (assuming it's a slave...is it?) will do the trick. Then you'll get back a string that is fairly easy to decode into an array. I can make an example for you if you specify whether the unit is a slave or a master and what registers you want and do you need to read or write them?
If you need to do more, buying a third party modbus library or building one yourself is the solution. Serial VIs or TCP/IP VIs will be at the bottom of the necessary code, but on top you need code to generate and decode modbus messages. If buying is the preferred way to go just try searching for modbus driver for LabVIEW on the net. I know you can find one at www.cardiac.no, but that only supports serial modbus as far as I know. If you want to get to know the protocol check out the documents available at www.modbus.org
