The demo data already warns it will wipe your site with the demo data in it. That is good for a starting point on your site.
The default user and pass are in your customer account in sitemile.com
after successfully importing the demo data, use that user and pass and login for the first time to the admin. Then add your own admin user, and delete the default one. If you cant handle that, we have our team that can do it for you. Contact us http://www.sitemile.com/contact-us