Two things immediately jump to mind:
1) the VISA drivers are not setup right. Reload the drivers and try again.
2) non NI VISA drivers and MAX do not get along. With our HPIB cards, we have to setup the VISA to "Allow Undefined Names" and then use names like "GPIB0::19" where the "19" would be the GPIB bus number of the instrument. If you use this naming format, most GPIB cards will work and the program will work with NI GPIB cards as well as others (we have 3 different types in-house and our LabVIEW programs do not have to be rewritten for each card).
