Don't get me wrong, I like your product and I'm *very* close to commit myself on it. But I had to dig in multiple places to get a view of native dll interfaces. Creating a window in pure Win32 model (Petzold style) with your dll is easy. Just need to know the class name and then it is only a matter of finding the right messages to send or expect to / from the window to interact with it. Short of a minor detail which I'm still debugging today, I could even build a control class in MFC which encapsulate pretty well the visual control.
But I had to dig in your C# interface to find most of my answers. Maybe the documentation might see some improvements in a future version. Again don't get me wrong, I'm mostly impressed with what I have seen.