To find those Mega Stones, you need to have upgraded the Mega Ring and look for the stones between exactly 8 and 9 PM. Look for something sparkling on the ground.
To upgrade the Mega Ring, go to Kiloude City. Fight several rounds in the Battle Mansion, and then go to the north end of the city. Fight your rival, and he/she will explain how to upgrade the Mega Ring. (You'll also get Absolite when you beat your rival.)