If the unit has a driver in the form of a dll you use a code library function node to call that dll. You can find lot's of information and examples on calling dlls in LV. If there is an activeX driver there's activeX functions you can use to call that. (If nobody else has made a "wrapper"-VI for the driver before consider sharing it with others if you make one...:-))
Sometimes USB units come with a driver that when installed transparently presents the unit as soemthing more familiar e.g. a standard COMport. In those cases you can use the standard ways to communicate with such a unit; in the mentioned case the serial VIs (old or VISA-based) will work fine.
