Hello
Sorry to say it is not possible to install the demo installer to an existing Joomla installation. The 'Demo Installer' is basically a fully functional Joomla! install package containing the CMS itself, your template and accompanying modules, the template demo images, all the configurations and site content that we used on the demo.
Details on installing the demo installer are all available in the 'Setting up the Demo Installer' tutorial..
http://www.joomla51.com/tutorials/item/setting-up-the-demo-installer
Ciarán