Using Disqus on a WordPress.com site requires a WordPress.com Business plan which supports third-party plugins like Disqus.
To install a plugin from the Plugin Directory, head to My Site → Plugins, and click the Add Plugin button. Search for "Disqus" and select the plugin authored by Disqus. Click "Install" and the plugin will be added to your site.
For more information on adding Disqus to your WordPress.com site, check out WordPress' documentation here.