In that case your problem is stemming from there.
I your DLL doesn't have a DLLInstall entry point (and that is not mandatory to have) then you won't be able to reference directly from your C# application.
You'll have to use the P/Invoke mecanism to interact with your COM DLL from the C# application:
static extern void SomeMethod(....);
You will need to define a external static method in you C# application for each COM method you want to use. But if you want to be able to reference your COM DLL then you should make sure it has DLLInstall and DLLUninstall method.