In Pokemon Pearl, you can complete the entire Pokedex by going through regular play and then going to Celestic Town and reading the book in Cynthia's grandmothers house. It will have a picture of Dialga. In Diamond, it's the same way but reversed (you'll see a picture of Palkia, rather than Dialga.). To find Rotom, go the Old Chateu in Eterna Forest between 8pm and 4am and go through the center door and go down the hall to one of the rooms. One of the rooms will have a TV you can examine and Rotom will come out.
There are other Legendaries in Platinum you have to obtain by special means, but the Sinnoh Pokedex for Platinum is greater than 150 so I won't bother explaining them all to you.
Lemme know if you have any other questions ^_^ I know a lot about Diamond, Pearl, and Platinum.
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150 hope i helped :) Actually there are 151 pokemon in the Sinnoh Pokedex but you only have to see 150 to get the National Pokedex.
There is only one legitimate way to complete the Sinnoh Pokedex. To complete the Sinnoh Pokedex you must trade for the missing Pokemon with Pokemon Diamond, Pearl or Platinum.
Please see the related link for a complete Pokedex from the Sinnoh region.
Just go to Pokemon platinum central and click on pokedex and it shows you where to battle all the Pokemon in sinnoh.
First complete the sinnoh pokedex, 210 Pokemon