The Relocator is a menu option that can be obtained via "codes." First, go to Castelia City (west of Nacrene City and home of the third gym leader, Burgh) and head to the most northern area of the city (Hint: Head north of the water fountain). There will be an entrance to a building on the left side of the area. Enter it and head into the elevator. Select 11F and you will walk into a room with a clown and a scientist. Talk to the scientist and he will ask you if you know some special words. Answer him with the following words.
Answer 1: "Everyone_Happy"
Answer 2: "Simple_Connection"
He will give you the special Relocator invention he made. To use this option, go back to the main menu and select the option for the Relocator.
The Relocator will transfer Pokemon White & Black event Pokemon from your Pokemon Pearl, Diamond, Platinum, Heart Gold, or Soul Silver to your version of Pokemon Black or White. These event Pokemon include the special Entei, Suicune, Raikou, and Celebi which are obtained at special Pokemon events.