How can I see all (active) clients on my Airport Extreme. I can’t find such option in the current Airport Utility.
If your need is a list of the wireless clients you can see them in the current (6.3.x) Airport Utility; hold down ALT and double-click the Extreme.
You can do it by installing an older version of AirPort Utility. Alternatively, you can use 3rd-party software…
iNet will scan your network for all connected devices and show you a list of devices with their hostnames and IP addresses. It also has a dedicated mode for AirPort routers showing devices connected and relevant information about each one.