This is because you have not used the Battle Maison before. For your rival to appear, you must have participated in at least one round in the Battle Maison; if you don't do this your rival will never appear.
Once you have played in the Battle Maison, go to the pond area at the top of the area. Your rival will be there; defeat her or her, and go to the Anistar City sundial to get your Mega Ring upgraded.
Once that's done, you will be able to find the hidden Mega Stones! Remember that they will only appear if the time on your 3DS clock is 8 to 9 PM.