Be sure to press and hold the reset button for sufficient time for the LED to blink rapidly, then release it. It will take five to ten seconds. It will then illuminate solid amber for about a minute, after which it will flash amber, slowly. That is when AirPort Utility should be able to recognize it.
You should feel, and may hear, a barely perceptible click when you press the button. There are anecdotal reports of users who may have damaged the reset button by pressing too hard, so don't do that.