Mega Pokémon
Only Pokemon which can mega evolve in Pokemon X and Y can mega evolve on Pokeheroes. This does not include Pokeheroes event Pokemon. Only a Pokemon with a Mega Bubble icon on it's summary page will be able to mega evolve.
How to get Mega Pokemon
Only Pokemon on this list are capable of mega evolving.
A Pokemon may randomly hatch out from a normal egg with a 'Mega bubble' on it's summary page. Some Mega Powered Pokemon are/were obtainable through special events, Such as; Mega Powered X and Y Mewtwo, Mega Powered Torchic, and Mega Powered Charizard X and Y.
Mega Evolving Event Pokemon
The only PH event Pokemon which can be mega evolved are Spring Ampharos and Summer Ampharos.
Lucario-sensei and Festival Gardevoir may hatch with a mega bubble on their summary page, but currently cannot be mega evolved.
How to get a Mega Stone
There are currently two ways to obtain a Mega Stone. You may get one by boiling it in the Gem Cauldron. It is the rarest item (and the hardest) to get from the Gem Cauldron. You can also find one when playing the Treasure Hunt game in the Game Center. Some Mega Stones can also be obtained through special events in the future.
How to mega evolve Pokemon
To Mega Evolve your Mega Powered Pokemon, you will need to give it a Mega Stone to hold. (This can be done through the item bag) Once the Pokemon reaches level 50, it will evolve and the Mega Stone will disappear. Mega Evolutions are not reversible