How do you complete the hoen pokedex?

that is only possible with a gameshark or a cheat.

Catch/hatch/trade/evolve all of the pokemon.

In Pokemon Platnunim you will talk to the professer and you will get Zaptos and moltrase aticuno and aceurs. in heart gold you go to this guy (no idea where) and he will give you the nationaldex...

A diploma, A gold trainer card.

well you can iether do it the hard way and battle lots of trainers and walk in tall grass until you see every Pokemon in the sinnoh regeion, or u can be awesome and use the action replay cheat to...