If you're writing a C DLL that you expect to be able to call from VB, you may be tempted to use the __declspec(dllexport) keyword. While this works fine for having other C functions call your DLL, VB is unable to find the exported functions. Instead, you should export your functions using a .DEF file.
The structure is pretty easy. You tell it you're writing a DLL, give it a name and description, and list the exported functions, with optionally a numeric ordinal.