First you must win your first gym badge at Striaton City. After the battle make sure you heal your Pokemon at the PokeCenter. When you leave the poke center head to the right until you enter the next area (where you receive one of your Pansear variations). Use cut, and enter the door way. You will then see an event involving Team Plasma. After that head back to Fennel's house and he will give you the C-Gear equipment. After that you will have access to the Union Room.
Chat with our AI personalities
in the union building
To change your appearance in Pokemon Platinum's Union Room you need to talk to an NPC in the Oreburgh Pokemon Center. The NPC in question has the appearance of a Psychic trainer.
yes
You trade in the union room with another person.
In the Pokemon League in the Champions room.