They should have come with the kit and VISA comes with LabVIEW (see the device driver CD). If you don't have the CD, they are downloadable from www.ni.com. I am not sure which OS you are using, but just go here to find them...
For NI-VXI, which is the required device driver to use the MXI-2 interface
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BE600704D8D?opendocument&node=132060_US
For VISA, go here
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BBB002C0E91?opendocument&node=132060_US