WordPress.org

1. Download the plugin archive and expand it (you've likely already done this).
2. Upload the 404-notifier.php file to your wp-content/plugins directory (not in a sub-folder).
3. Go to the Plugins page in your WordPress Administration area and click 'Activate' for 404 Notifier. This will create the 404 log table for you.
4. Optional: Configure your 404 Notifier options and get the RSS feed URL by going to Options > 404 Notifier in the WP Admin area.
5. Optional: Subscribe to the 404 Notifier RSS feed (link on the options page).

Congratulations, you've just installed 404 Notifier.