Once the creature has been unlocked he will become available to you the next time you run Black & White - however finding him is a different matter. Take note though, to take control of your new beast you must already have chosen a creature from the game.
Once you have chosen your creature you may exchange him immediately if you wish, but to do so you must locate the whereabouts of the Creature Breeder. This is not as easy as it sounds, for he only appears on Land 1 and Land 4, and even armed with that knowledge you will still have to find his hut. I will tell you this though; breeding creatures is a lonely business, so he tends to stay away from village life by making his home in one of the more isolated parts of the land.