WordPress.com Popular Posts can be installed easily:
- Download and install Jetpack
or WordPress.com Stats plugin.
- Download WordPress.com Popular Posts .zip archive
- Extract the files in the .zip archive, and upload them (including subfolders) to your /wp-content/plugins/ directory.
- Activate the plugin through the 'Plugins' menu in WordPress
- Inside the WordPress admin, go to Design > Widgets, and add the 'Popular Posts' widget where you want, then save the changes.
- If you want, you can customize some settings for the widget, in that page.