Your Pokémon can now use their Mega Evolution in Pokémon Go. Not all Pokémon can do this. So far, only Charizard, Blastoise, Venusaur, and Beedrill can go through the transformation. You won't be able to make them use their Mega Evolution when you want to use it. You can only use it if this Pokémon has enough Mega Energy.
You collect Mega Energy by participating in Mega Raids. These are different from the traditional Raids you've come across in Gyms around the world. The Pokemon you have to battle in the Mega Raid is a Mega Evolved Pokemon, so it's likely stronger than the Raid Pokemon you encountered with your friends. The faster you complete the Mega Raid, the more Mega Energy your Pokemon collects to use for its Mega Evolution.
After Pokémon Mega Evolve, it will take less Mega Energy to go through this transformation every time you need it. The Mega Evolved Pokémon will have increased stats, making it a great asset for going in raids with your friends or battling Team Rocket. You can make your Mega Evolved Pokémon your friend to explore the world with you and take pictures. Still, you won't be able to use them in the Pokémon Go Battle League when it launches.
It looks like you need to use the Pokemon you want to Mega Evolve in the Mega Raid to collect energy. Be sure to use the Pokemon you wish to Mega Evolve to battle in the raid.
