Reptile constructed a prototype while at the abandoned Adventurers outpost in the Desert region of Adventurers' Island, to aid the Desert team in finding the Maelstrom. He then built three more, so that they could be distributed to scouts.
When the Desert Team entered the Jungle, Reptile gave all four devices to four scouts, including Agent Fireball, and sent them out to find either the Maelstrom or Rex and the others. Fireball found neither- instead, he stumbled onto a massive fortress guarded by henchmen of Dr. Rex. His suspicion was that the fortress was near the Maelstrom and contained Maelstrom tapping equipment, and this was verified through the sensor. Fireball was then chased off by guards, and hastened to report these findings to Reptile.
Reptile then modified Fireball's Dark Energy Detector to be a little more accurate, and used it to retrace Fireball's path to the fortress, accompanied by a dozen other agents. They agreed with Fireball that it was the center of Dr. Rex's operations in the Maelstrom, and set about planning an attack.
The Dark Energy Detector does not actually scan for the dark energy generated by the Maelstrom, as the name suggests. In fact, it scans for any radioactivities and energies leaking through rips in the space-time continuum. The Maelstrom merely happens to be a prime example of such an anomaly. It's just a cool-sounding name.
The various sensors gather in data, which is processed and outputted to a small display as a number. If the number is 100 or greater, the Maelstrom or another anomaly of equal strength is nearby (approximately within a kilometer). If somewhere between 70 and 100, the anomaly is anywhere from one to five kilometers away. If between 50 and 70, there is an anomaly but it could be up to ten kilometers away. And if less than 50, there is not necessarily an anomaly nearby (it could just be background radiation).