WordPress.org

1. Upload the file widget-feedburner.php to your WP plugin folder /wp-content/plugins/ directory
2. The path must look like this: /wp-content/plugins/widget-feedburner.php
3. Activate the FeedBurner Widget plugin through the Plugins menu in WordPress admin
4. Go to Appearance: Widgets, Add widget FeedBurner Stats and Save Changes
5. After the page reloads on the widget FeedBurner Stats at right, click Edit, set your FeedBurner Username for the feed URL and customize the options you need in the widget settings
6. Click Done and Save Changes to the sidebar from the button bellow
7. You are ready, check the FeedBurner readers button in the sidebar of your favorite WordPress blog

Good luck!