Welcome to Player Attack!
You can grab the new Pokémon now via WiFi - all you need to do is launch the game while you're connected to the internet - the main screen will give you the option to receive a "mystery gift". Obviously, it's not that much of a mystery - head to any in-game Pokémon Center and speak to the Delivery Girl, she'll hand over your new Patter Vivillon.
Whatever you do, be quick! It'll disappear again on August 12!
Jimmy the Geek