The National Dex

Pokemon Platinum Walkthrough and Guide by Mogg 13-42  

Pokemon Platinum Guide - Sidequests

The National Pokedex

Twinleaf Town & Sandgem Town
Items: National Dex, Pokeradar.

After beating Cynthia, you'll be taken back to Twinleaf Town. Now, if you
managed to meet all 210 Pokemon in the Sinnoh region, head to Sandgem Town
and proceed to Rowan's lab. Show him your completed Pokedex and he will
reward you with the POKE RADAR application for your Poketech. Professor Oak
will come in and upgrade your Sinnoh Dex to the National Dex.

Head on over to our Pokedex for assistance in trying to find any Pokemon you are missing.

The members on our forum are also usually able to help.