Now that the main game is over, the post-game begins. You'll find yourself in the place where the game originally began: your bedroom in Twinleaf Town. When you go downstairs, your mother will mention that Barry was just over and mentioned something about catching a ship at Snowpoint City.
By now, you should have all 210 Pokémon in your Sinnoh Pokédex sighted and in Sandgem Town you can receive the National Pokédex. If not, you may have missed some. Here is a list of regional Pokémon not owned by a Trainer in Sinnoh.
Manaphy: Picture found in a book on a desk in Mr. Backlot's office in his mansion on Route 212.
Mesprit, Azelf, and Uxie: Found in their caverns at their respective lakes after the Giratina battle in the Distortion World.
Dialga and Palkia: Counted as seen during the story event at Spear Pillar.
If you are ready, head to Sandgem Town.
Once Professor Rowan confirms that you have seen all the Pokémon in the Sinnoh Pokédex, Professor Oak will appear. Having heard of your exploits, he will reveal he is bringing components for the National Pokédex and your Pokédex is automatically upgraded. Before he leaves, he'll mention that Pal Park on Route 221 is open. Now you can import Pokémon from the GBA games.
Professor Rowan will then give you the Poké Radar, which will allow you to find non-regional Pokémon in tall grass. For it to work, you need to be standing in tall grass without being mounted on your bicycle. After one use, it takes 50 steps to recharge.
Once you have the National Pokédex, a swarm of Pokémon from a different region will appear on a different Route each day. The house south of Professor Rowan's lab belongs to Lucas/Dawn. Their little sister can tell you which of the swarm Pokémon is currently present.
By playing your Platinum game with a Generation III cartridge inserted in your Nintendo DS or DS Lite's GBA slot, you can make certain Pokémon appear on certain routes and areas throughout Sinnoh. Most of the Pokémon that appear depend on which GBA game is inserted, but Haunter and Gengar will appear in the Old Chateau if you insert any Generation III game.
If you want to catch a lot of rare Pokémon, you should visit the Great Marsh at the north end of Pastoria City. For 500 you can explore the Marsh until you throw 30 Safari Balls or walk 500 steps, whichever comes first. You won't use your Pokémon to battle here - if a wild Pokémon appears, you can throw Mud, Bait, or a Safari Ball. Bait lowers the chance of the Pokémon escaping, has a 90% chance of lowering its catch rate. The mud does just the opposite, making the Pokémon easier to catch, has a 90% probability of causing its chances of fleeing to rise.
For 100, you can look through the binoculars in the upstairs room of the entrance building before a Safari Game. They show wild Pokémon in the Marsh area where they can be found. This is useful for spotting the rare Pokémon that are randomly selected each day.