Go in front of the Pokemon Center, then go into the street that is to the right. When you go into the street, look at the first building to the right and go inside it. You'll see a lady and Zorua. The Zorua is friends with the event Celebi that you've got to have. Otherwise you can't get Zorua. Also make sure you have a pokeball (any) and an empty space in your party.
Hope this helped you!!