We've installed and activated plugins in previous lessons. If you need help, go back to the plugins lesson and download/upload and activate the Posts by Author plugin.
This image just shows you the plugin page, and note that it indicates a link to download the plugin for the latest version of WordPress. Download to your DemoTown Folder, unzip it, and then upload to the wp-content/plugins folder on your site. Go into the plugins page and activate it, then check the next step for setup.