Domain Mapping Not Working - where to go?

Hi:

I redirected a domain to the IP address of the subdomain.mysite.com address. I then created a mapped domain using the WPMUDEV domain mapping plugin. When I try to access the site using the mapped domain address I get a 404 error - requested resource not found. Not sure where to look to find the cause of this error. Any suggestions would be appreciated.

FYI: Wordpress is running on IIS 8.5 on windows.

Thanks!