Question

How can I register tornadrous, vulbaby and mandibuzz in Pokemon white?

Answer 1

You must either battle people with those Pokemon or Trade to get them.
I think Pokemon Dream World or Dream Radar can get you those Pokemon.

Hope this helps

Answer 2

In order to register those Pokemon in your Pokedex, you must ether 1, battle someone with those Pokemon, or 2, trade via wi-fi or with a friend obtain those Pokemon. Hope I helped.