This one is complicated, so steps are the way to go.
- Make sure you have your Mega Ring upgraded (to do this, go to the Battle Maison in Kiloude and use it for a while, then meet your rival in the top of the city. Beat them and find Professor Sycamore at the Anistar City sundial and he will upgrade your ring.
- Make sure the time on your 3DS is 8:00 PM - 8:59 PM. If it is not, you will not find the Mega Stone.
- Go to Route 22. If you look around for a while you will find a trainer who is standing nearby a lake. Defeat her if you haven't already and use Surf. There will be a waterfall; go down it. You should reach an island with a cave on it. Enter the cave and look for a sparking object. That object is the Banettite.