Hi,
When writing a GPIB program in C++ you will need to attach the ni488.2.h file. I think the best thing for you to do is reference the shipping examples which can be found in the following files. C:\Program Files\National Instruments\NI-488.2\Languages. In this file you will find examples for several different programming languages. Please look at these they will show you proper programming for GPIB and how to include the proper files.
Regards
Krista S.
Applications Engineering
National Instruments