Best would be to write a 'DDraw' test (integrated in the test suite) to be able to check reference counting in some 'standard' cases (surface creation, D3D object creation, ...).
I am afraid this will have to wait a few days, as I don't have access to any Windows machine the next days.
Well, as we call a DDraw 'method' to create the surface, you should add the AddRef call there. For the release, it should be in the 'final_release' call for the surface (AFAIK, each surface already stores a pointer to its parent DDraw object).
How about this one? It increases the refcount in Main_DirectDraw_AddSurface and decreases it in Main_DirectDraw_RemoveSurface, the places where the surface is attached / removed from the DirectDraw7's surface list.