WordPress.org

The plugin is simple to install:

1. Download the zip file
2. Unpack the zip. You should have a directory called ajax_calendar, containing several PHP files
3. Upload the ajax_calendar directory to the wp-content/plugins directory on your WordPress installation. It is important you retain the ajax_calendar directory structure
4. Make any modifications to your theme, as required
5. Activate plugin

You can find full details of installing a plugin on the plugin installation page.