Bottom-left in eterna
The Pokeradar is an item that you get in Pokemon Platinum. You get it from Professor Oak after completing the Shinoh Pokedex.
Speak with Professor oak he will tell you about them and then they come
Professor Oak Is The Professor From Kanto(The Original Professor) Who Came To Sinnoh Because He Heard That Articuno,Zapdos,And Moltres Had Come To Sinnoh.
he doesn't
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
get all the Pokemon in your pokedex from sinnoh then talk to professor rowan and oak will give it to ya
Complete the Sinnoh dex then go to professor rowen for it professor oak, he will enter the room and give you it.
You can trade from Platinum to SoulSilver as soon as you have received the regional Dex from Professor Rowan in Pokémon Platinum and as soon as you have received the regional Dex from Professor Oak in Mr. Pokémon's House in Pokémon SoulSilver.
Roaming around the main continent.
You have to see every pokemon in the regional pokedex. Then you have to obtain the national pokedex from i think professor oak or professor rowan.
You can get him in Pal Park or you can trade with a friend who has Heart Gold and got Squirtle from Professor Oak.
I believe you meet him after completing the Sinnoh Pokedex. Then he will update your pokedex to national.