The STUN Server plugin provides address discovery for peer-to-peer sessions to be used for media transmission and receiving of UDP packets. It's especially useful for clients behind NAT devices. More details on STUN is available on http://en.wikipedia.org/wiki/STUN.
Copy the file, "stunserver.jar" into the plugins directory of your Openfire installation. The plugin will then be automatically deployed. To upgrade to a new version just replace the "stunserver.jar" file with the newest version or you can just update the plugin from the admin console in the plugins page.
Once the plugin has been installed you can log into the admin console to modify the STUN server settings.
Log into the admin console and in the main page you will see the Media Settings tab. In that tab, you will find the STUN settings menu item. Go to that page to configure the STUN server.