Probably this is because you did not indicate this entry point should be exported from the DLL.
Consider
__declspec(dllexport) STDAPI DllUnregisterServer(void) { }
I suggest you check up what's exported using some binary dump utility such as
dumpbin.exe
,
http://msdn.microsoft.com/en-us/library/c1h23y6c(v=vs.71).aspx[
^] (to use this utility, it's best to start from "Visual Studio Command Prompt").
—SA