Using var rather than function in the prototype doesn't change it. DLL_USE is in there:
#define WIN32_LEAN_AND_MEAN // Exclude rarely-used stuff from Windows headers
#include <windows.h>
#define DLL_USE // always define before including adll.h
#include "adll.h"
I don't understand what's going on because I have written plugins for A6 in the past, and they always worked fine.
Edit: Okay, I fixed it. I wasn't doing it quite properly when I originally put the DLL in the game folder. It's fine now.