Have you upgraded your Mega Ring yet? If not, you can do it by going to Kiloude City and defeating Calem/Serena (which you can do after fighting the Battle Maison a few times). You'll get an Absolite, and you can then meet Prof Sycamore in Anistar City. He'll upgrade your Mega Ring, and after that you'll be able to find the night Mega Stones if the time is anywhere between 8 and 9 PM.
After that, you should be able to find those Mega Stones. Their location will sparkle, so they are hard to miss.