Here it Goes,
After beating the Elite 4 and the Champion, you have to go to Kiluode City, To get to Kiluode City you have to beat the E4, Diantha and AZ you'll be in your house and when you Exit your room, Grace (Your Mom) will tell you to go to Lumiose Station in Lumiose City to go talk to Prof. Sycamore. If you don't know where to go Then go into the Lumi Cabs and the Guy will take you to the station go in There, then Sina and Dexio will Upgrade your Pokedex to National Mode. After that talk to Prof. Sycamore then he'll give you the Key Item AMV Pass to go to Kiluode city the go on the Train then You'll be at Kiluode city. Win at Least one Battle in the Battle Masion then at the top of the City Your Rival Serena (If your Male) or Calem (If your Female) will be there to battle you. Beat Him/Her and He'll/She'll give you the Mega Stone Absolite and then He'll/She'll tell you to meet Prof. Sycamore in Anistar City by the Pink Sundial. Go to Anistar City and Talk to Him and He'll Upgrade your Mega Ring and you can now find all The Mega Stones at you own Leisure.
Source: Experience
Hope this helps and Good Luck :)
