Instructions
Fight every single Pokémon Trainer in "Pokémon Platinum." Your Sinnoh Pokedex will automatically register every Pokémon you use during battle. If you skip a Pokémon Trainer, return to fight her as soon as possible.
Walk in the long grass on every route. Every new wild Pokémon you see will contribute to your Pokedex. Don't use a Repel when traveling through caves, certain buildings and other places where wild Pokémon appear. Since there are a variety of Pokémon on each route, you should encounter 10 to 20 Pokémon before you leave a location.
Go to the Solacean Ruins, which are east of Solacean Town. Take the following exits to get to the last room of the ruins: the top right, lower left, top right, top left, top left and lower left. Once you reach the last room, you will activate the Unown. Go to any other room of the Solacean Ruins and walk around to encounter an Unown.
Travel to Route 212 and go inside Mr. Backlot's Mansion. Find Mr. Backlot in the main room of the mansion. Walk up to the desk and press "A" to examine the book. You will see a picture of Manaphy, and your Sinnoh Pokedex will register Manaphy. No trainers in this game have Manaphy, and this Pokémon does not appear in the wild.
Enter the Old Chateau in Eterna Forest. Go upstairs and go through the middle entrance. Walk to the door to the left of the one your character is standing before. Save your game and inspect the TV. If the time is between 8 p.m. and 2 a.m., a Rotom will appear. This is the only Rotom you will battle in the game.
Check your Sinnoh Pokedex to see if you have 210 Pokémon. If you do, go to Professor Rowan's laboratory in Sandgem Town. Speak to him and Professor Oak will enter and interrupt your conversation. Upon hearing that you completed the Sinnoh Pokedex, Professor Oak will give you the National Pokedex.