When you define a NET interface along with a ComVisible class with your interface set as the default one , VB6 hides this default interface entirely from you. Then, when you want to work with that interface in VB6, you use the name of the implementing class.
The Default interface is set either with ComDefaultInterface attribute or if this is left out, C# will make the default interface the first Interface that the Class implements.
So if you want to see the interface in VB 6.0 you should not have any class implement directly the default interface, but an interface that derives from the default one. Remember that for COM all the interface members of the base interface should be redefined in the derived interface using the new keyword.