mPixel.net – This is a live site where you can purchase pixels and post a link to your site.
This is not a demo. There is currently no demo where you can test the admin interface.
If you like you can set up your own demo. You can set up a webserver on your computer using this software.
Then once you have your server set up simply create a database and install MDS into it. Follow the steps in the documentation for more info.