You must get to Kiloude City before getting the Mega Ring.
After beating the Elite Four/Diantha, go to Kiloude City via the TMZ Hi-Speed Rail in Lumiose City.
Then, after using in the Battle Maison in Kiloude, your rival is near the top of the city. Once you defeat him/her in battle, she will give you the Absolite and tells you Professor Sycamore is waiting for you in Anistar City by the Sundial.
Once you go talk to Sycamore, he tells you about the upgraded Mega Ring and upgrades it for you.
Once it is upgraded, you can hunt for Mega Stones between 8-9 PM on your 3DS time.