If you want plain C exports, use a C project not C++. C++ DLLs rely on name-mangling for all the C++isms (namespaces etc...). You can compile your code as C by going into your project settings under C/C++->Advanced, there is an option "Compile As" which corresponds to the compiler switches /TP and /TC.
If you still want to use C++ to write the internals of your lib but export some functions unmangled for use outside C++, see the second section below.
Exporting/Importing DLL Libs in VC++
What you really want to do is define a conditional macro in a header that will be included in all of the source files in your DLL project:
#ifdef LIBRARY_EXPORTS
#    define LIBRARY_API __declspec(dllexport)
#else
#    define LIBRARY_API __declspec(dllimport)
#endif
Then on a function that you want to be exported you use LIBRARY_API:
LIBRARY_API int GetCoolInteger();
In your library build project create a define LIBRARY_EXPORTS this will cause your functions to be exported for your DLL build.
Since LIBRARY_EXPORTS will not be defined in a project consuming the DLL, when that project includes the header file of your library all of the functions will be imported instead.
If your library is to be cross-platform you can define LIBRARY_API as nothing when not on Windows:
#ifdef _WIN32
#    ifdef LIBRARY_EXPORTS
#        define LIBRARY_API __declspec(dllexport)
#    else
#        define LIBRARY_API __declspec(dllimport)
#    endif
#elif
#    define LIBRARY_API
#endif
When using dllexport/dllimport you do not need to use DEF files, if you use DEF files you do not need to use dllexport/dllimport.  The two methods accomplish the same task different ways, I believe that dllexport/dllimport is the recommended method out of the two.
Exporting unmangled functions from a C++ DLL for LoadLibrary/PInvoke
If you need this to use LoadLibrary and GetProcAddress, or maybe importing from another language (i.e PInvoke from .NET, or FFI in Python/R etc) you can use extern "C" inline with your dllexport to tell the C++ compiler not to mangle the names.  And since we are using GetProcAddress instead of dllimport we don't need to do the ifdef dance from above, just a simple dllexport:
The Code:
#define EXTERN_DLL_EXPORT extern "C" __declspec(dllexport)
EXTERN_DLL_EXPORT int getEngineVersion() {
  return 1;
}
EXTERN_DLL_EXPORT void registerPlugin(Kernel &K) {
  K.getGraphicsServer().addGraphicsDriver(
    auto_ptr<GraphicsServer::GraphicsDriver>(new OpenGLGraphicsDriver())
  );
}
And here's what the exports look like with Dumpbin /exports:
  Dump of file opengl_plugin.dll
  File Type: DLL
  Section contains the following exports for opengl_plugin.dll
    00000000 characteristics
    49866068 time date stamp Sun Feb 01 19:54:32 2009
        0.00 version
           1 ordinal base
           2 number of functions
           2 number of names
    ordinal hint RVA      name
          1    0 0001110E getEngineVersion = @ILT+265(_getEngineVersion)
          2    1 00011028 registerPlugin = @ILT+35(_registerPlugin)
So this code works fine:
m_hDLL = ::LoadLibrary(T"opengl_plugin.dll");
m_pfnGetEngineVersion = reinterpret_cast<fnGetEngineVersion *>(
  ::GetProcAddress(m_hDLL, "getEngineVersion")
);
m_pfnRegisterPlugin = reinterpret_cast<fnRegisterPlugin *>(
  ::GetProcAddress(m_hDLL, "registerPlugin")
);