Check out the following tutorials:
http://zone.ni.com/devzone/conceptd.nsf/webmain/6792BAB18242082786256DD7006B6416?opendocument&node=200065_US
http://zone.ni.com/devzone/conceptd.nsf/webmain/67E175B2AF9AA10286256A9D0078E2E7?opendocument&node=200065_US
Normally it's best to use drivers accompanying the USB equipment but you can also use VISA to communicate directly, that requires that you know the protocol of the unit though.