The Pokemon added to the Sinnoh Pokedex in Platinum are:
Rotom
The Gligar Line (Gligar and Gliscor)
The Nosepass Line (Nosepass and Probopass)
The Ralts Line (Ralts, Kirlia, Gardevoir and Gallade)
The Lickitung Line (Lickitung and Lickilicky)
The Eevee Line (Eevee, Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Leafeon and Glaceon)
The Swablu Line (Swablu and Altaria)
The Togepi Line (Togepi, Togetic and Togekiss)
The Houndour Line (Houndour and Houndoom)
The Magnemite Line (Magnemite, Magneton and Magnezone)
The Tangela Line (Tangela and Tangrowth)
The Yanma Line (Yanma and Yanmega)
Tropius
The Rhyhorn Line (Rhyhorn, Rhydon and Rhyperior)
The Duskull Line (Duskull, Dusclops and Dusknoir)
The Porygon Line (Porygon, Porygon2 and Porygon-Z)
The Scyther Line (Scyther and Scizor)
The Elekid Line (Elekid, Electabuzz and Electivire)
Th Magby Line (Magby, Magmar and Magmortar)
The Swinub Line (Swinub, Piloswine and Mamoswine)
The Snorunt Line (Snorunt, Glalie and Froslass)
Absol
Giratina