Ok, I gave up on trying to get IIS to work on my Win10 machine and instead installed XAMPP and was up and running in a short time. Extracted core grav + admin. Everything went smoothly … up until I tried to use the Admin Plugin/Themes options. I get an error. GPM not reachable. At least I think I know what the problem is here - my machine sits behind an authenticated proxy. This means I’ve got to pass a username and password to it. How do I do that in the field Proxy URL?
Fill the proxy options in the System configuration, by adding the username and password in the http://username:password@proxy.thing.com:8080/ shape.
Also check out this article in the Learn site if that does not help.