fatal error LNK1120: 1 unresolved externals

I'm using C++6.0 and DirectX 7.0 with the SDK.

I'm trying to do this:
hRet=DirectDrawCreateEx(NULL, (LPVOID*)&lpDD,IID_IDirectDraw7,NULL);

it seems the compiler can not find IID_IDirectDraw7 and gives this error:

--------------------Configuration: Life - Win32 Debug--------------------
Linking...
LifeView.obj : error LNK2001: unresolved external symbol _IID_IDirectDraw7
Debug/Life.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

Life.exe - 2 error(s), 0 warning(s)

under project settings->lib I have this: ddraw.lib guids.lib

What is going on? Along with the answer could someone please tell me what an unresolved external symbol is and how I can solve this problem on my own eg. where to find the symbol. Thanks a lot!