To get Zorua, you will need to have first transferred Celebi over to Black & White using the Relocator Machine unlocked in Castelia City. Once done, take Celebi to one of the buildings in Castelia City where you'll see a girl with another girl who is apparently lost. Celebi will come out from it's Pokeball and the lost girl will shed her illusion and turn out to be Zorua. Zorua will be offered to join you. However, you do need a Pokeball as one will be used up when you obtain it. It can be any ball.
Added by: nyan_nyan_pyui_123 Aug 2nd 2012, ID#17270