The interview confirmed it's post game.I think it was they didn't make any new Mega Evolutions as they wanted to focus on Z-Moves (which to me is a slippery slope, if you completely focus on the new mechanics then that means previous new mechanics will never be added upon. GF is always introducing new stuff, so does that mean in Gen VIII that there won't be new Mega Evos or Z-Moves because they want to focus on the new mechanic... which would then be equally ignored in Gen IX for its new mechanic, etc.).
We're getting the Key Stone and a Mega Stone from Dexio and Sina and I'm assumed its during the story.
Also i found it
http://www.gameinformer.com/b/featu...for-pokemon-sun-and-moon.aspx?PostPageIndex=2o over the course of the main scenario of the main game, Mega Evolutions don't appear. But once you finish the story, you unlock the access to Mega Evolve Pokémon. You run through the main game learning about these new Z moves, and after the game you unlock more strategic depth by combining Z move with mega-evolve Pokémon.