You have to visit the extension's page here:
www.joomlaworks.net/extensions/free-premium/disqus-comments-for-joomla-package, download the package and then through your Joomla! administration you go to Extensions -> Extension Manager.
You browse to the file through the Upload Package File box and click Upload/Install button.
In the extension's page above read this paragraph carefully in order to set up the plugin properly.
"
USING DISQUS ON YOUR JOOMLA! WEBSITE
After you signup for an account and add your website or blog at the Disqus dashboard, all you have to do to integrate Disqus in your Joomla! website is copy your subdomain name (shortname) over to the (content) plugin's related field."
