If you are using an instrument driver for the power meter, you can put a reference to it by using the Project->References item in VB menus. You may have to browse to your DLL. Once you find it and reference it, you can call those functions directly in your program without having to declare anything else.
If you aren't using an instrument driver, you should follow the directions for your NI-488.2 software (that came with the PCI-GPIB) for how to use it in Visual Basic. I know that in the past they had a .bas file that you could just include in your project. I don't know if they still use that or if they also have a typelib you can "reference" like the instrument driver above. In either case, one you either include the .bas file or reference it, you can also
call those functions directly.
You probably don't need to do both. You will either communicate through the GPIB API or the instrument driver API and that's the DLL that you need to include a reference to.
