If you performed the "hard reset" correctly (read the following to be sure) and it still does not appear in AirPort Utility, then it may simply be defective.
Try the "hard reset" again. That's all you can do. If you still cannot get it to appear in AirPort Utility bring it to an Apple Store for evaluation, who will only do the same thing again before drawing a definitive conclusion.
- Press and hold the reset button with a pen or pencil until you see the status light (LED) start to flash amber rapidly, which should occur after about five seconds.
- Release the button, and the AirPort Base Station or Time Capsule will hard reset.
Close AirPort Utility 6.x if it is open.
With the AirPort Extreme powered on for a few minutes, perform a Hard Reset by holding in the reset button on the AirPort Extreme for 7-8 seconds. Release and allow a full minute for the AirPort Extreme to restart.
Click the fan shaped AirPort icon at the top of the screen and wait a few seconds for a listing of New AirPort Base Station to appear. (The example below shows an AirPort Express)
Just below that, click directly on the AirPort Extreme and AirPort Setup will open up automatically, analyze the network and suggest a configuration. Follow the prompts.
If you want to use your iPad to configure the AirPort Extreme......
Tap Settings on the home screen
Tap Wi-Fi on the left panel
Look for Setup a new AirPort Base Station on the right
Tap on AirPort Extreme and follow the prompts