AirPort base stations: Can't add additional AirPort base stations to an extended network if MAC Address Access Control is enabled
When configuring an AirPort base station to extend the range of your Wi-Fi network that has MAC Address Access Control enabled, the device is unable to connect after selecting Update. The status light on the additional AirPort base station blinks amber. You can no longer see the AirPort base station listed in AirPort Utility.
If you have MAC Address Access Control enabled on your primary AirPort base station with (default) set to No Access and you wish to add any Wi-Fi devices, including additional AirPort base stations, you must first add the MAC address of the Wi-Fi devices to the MAC Address Access Control list before configuring them.
Alternatively, you may disable the MAC Address Access Control setting by choosing AirPort Utility > AirPort > Access Control tab drop-down menu, then selecting Not enabled.
If you have already attempted to add an AirPort base station without disabling the MAC Address Access Control, you must perform a Factory reset of the additional AirPort base station using the reset button on the device. Then, use one of the suggestions above to reconfigure it.
For steps to reset your AirPort base station, see: Resetting an AirPort Base Station or Time Capsule FAQ.