Hi,
normally you will get this error, when NI-VISA is not installed, there is a KB on web for it:
http://digital.ni.com/public.nsf/websearch/6055CCB1C032D1DD86256C32004EC84D?OpenDocument
Else try to update your NI-VISA. You will find this here:
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BAC002C51FA?opendocument&node=132070_US
Rebecca Fox
Applications Engineer, Germany
National Instruments Germany