OS X Server: About the RADIUS Service
The RADIUS service is included in OS X Server (Mountain Lion) but it is not listed as a separate service in Server app.
OS X Server (Mountain Lion)
In OS X Server (Mountain Lion), RADIUS is enabled by selecting an AirPort device in the Server app sidebar and then selecting "Require user name and password login over Wi-FI". The Server app starts RADIUS on your server, registers the selected Apple wireless device with RADIUS, and authorizes all user accounts on the server (included users from connected external directories) to access your wireless network, using the server's default SSL certificate.
If you upgrade or migrate to OS X Server (Mountain Lion) from Lion Server or Mac OS X Server v10.6, your RADIUS configuration is preserved and remains active, although it may appear inactive in Server app until you Refresh the service (from the View menu). If you enable RADIUS as described above, it will overwrite your previous RADIUS settings.
RADIUS settings can be customized using the radiusconfig command. For instructions, see man radiusconfig.