Steps for OS X Lion or Mountain Lion
This can happen if IPv6 has been disabled. To enable IPv6 for your network interface:
- Choose Apple menu () > System Preferences, and then click Network.
- If a lock icon appears in the lower-left corner, click it and enter your admin name and password to unlock the pane.
- Click your network service (Wi-Fi or Ethernet) in the list, and then click Advanced.
- Click the TCP/IP tab.
- Choose Link-local only from the Configure IPv6 pop-up menu.
- Click OK.
- Click Apply.
The AirPort device should now appear in AirPort Utility.
For AirPort Utility for iOS steps, please see this article.