You can't. You have to get NationalDex and then talk to Bebe. She will give you a free eevee, though I think it only works once
cant YOU CAN MIGRATE IT FROM POKEMON FIRE RED OR LEAF GREEN AND GO TO THE PAL PARK AFTER GETTING THE NATIONAL POKEDEX
Dialga, Uxie, Mesprit, Azelf, and probly Arceas.
To catch Porygon in Pokemon Diamond you need the National Pokedex. After getting the National Pokedex go to Trophy Garden in Route 212 and talk to the owner. When he says he has seen a Porygon it will now appear in the Trophy Garden.
Okay, to get the Pokemon to fill the National Pokedex, either you: migrate Pokemon from Emerald, Ruby, Sapphire, Firered, and Leafgreen. Or you could go in the wild near thesurvival area, which won't help a lot but it can help. Hope this helps!!
Route 204 on Jubilife's City side. (after getting the national Pokedex insert FireRed cartridge into your DS)
You can do that by getting the national pokedex and you go to the house where you pass to go to Stark mountain.{you surf to the house}
To get Makuhita in Pokemon Diamond you first need the National Pokedex. After getting the National Pokedex talk to your rival's sister in Sandgem Town. She will let you know what Pokemon swarm is active for the day. Check with her everyday until she says Makuhita are active on route 225. Now simply go to Route 225 and catch yourself one.
To get a full National Pokedex, You need to use the Action Replay Chip thingy... I did it! It's really ez X-cept for getting the code because some people lie about the numbers on the code... >:(
You get the pokeradar for getting the National Pokedex
by getting the national pokedex.
Aerodactyl is found by digging up an old amber underground after getting the National Pokedex, and bringing it to the man in the Oreburg City Mining Museum.
you can obtain him after getting the second pokedex or you can switch him of emerald hope i helped