First, you need the Liberty Pass which gives you access to Liberty Garden to CATCH Victini.
The Liberty Pass is not obtainable through the Wireless feature at Gamestop. You get it via Wi-Fi through April something I believe.
On the start up screen go to Mystery Gift, and select get through Wi-Fi.
This will get you the Liberty Pass which will get you to Liberty Garden from the ship in the left most pier. Talk to that person and you will be taken there.
Victini will be Lv. 15, and I wish you good luck! ~