Unity Tower [...] is on an island in the southwest of the Unova region.
It is only accessible from the Unity
Pier in Castelia City after
registering a location in Geonet, and
making an international trade with
another user who has done the same.
EDIT: It also works even if you have the same language versions. One of my friends and I, we both have a German version of Pokémon White but I have Germany registered in Geonet and he has China registered in Geonet and by trading with him I got the Unity Tower on my map. After having visited the tower at least once you can also Fly there.