What is number 168 in Pokemon Platinum?

That would be Umbreon if you're looking for 168 in the Sinnoh Dex. The trainer who has it in their possession is on the south part of Route 212 - his name is Collector Dean.

or you could just grab the Eevee from Bebe in Hearthome City and train it and then level it up at night until it evolves into Umbreon. where abouts