You can use language interfaces or the Tools for Visual Basic (ActiveX controls) that are a component of Measurement Studio.
The language interfaces allow you do make calls to the GPIB drivers themselves. The ActiveX controls are what I prefer to use as it's an easier set up mechanism and hides some of the issues with just straight gpib calls to the driver ... a little higher level and easier.
You should be able to find information by visiting the Resource Library in NI Developer Zone: http://zone.ni.com/libraries/ and then by going to GPIB->Programming->Language Interfaces
There are examples there using the ActiveX controls and also with calls to the DLLs.
Regards,
Kamran