The Product Reviews free extension is available in Products > Extensions. The Product Reviews is the first one. Hit the install button to proceed with the installation process:
The installation will begin as soon as you hit the install button. Click the Activate Now button as soon as it is complete:
After you click the Activate Now button the plugin will be active and the new sub-menu will appear under products menu.
The Product Reviews system works similar to comments but it is enhanced with the rating, correct labels, separate screen to manage reviews and many other useful features.