I visited your profile to see which country you live in. If your profile is accurate, and you do live in England, then that is your problem. The Pokeball Vivillon pattern was only released as an event in North America, so unfortunately you will not be able to claim the gift.
To celebrate the grand opening of the Pokemon Center online store for U.S, the Pokemon Company has revealed fans will be able to obtain an exclusive Poke Ball Pattern Vivillon. Those who own a copy of Pokemon X or Y can download the special-patterned Pokemon via the Nintendo Network. However, this elusive Vivillon is only available to game owners in U.S. *
*It was later announced that this event is not only for players in the U.S. but for all players in North America.