Unfortunantly you cant go to other regions in Pokemon diamond and pearl. its not like leafgreen,firered,gold,silver and crystal version where you travel to different regions. You can't, Sinnoh is the only region available in the game.
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No you cannot go to other regions in Pokemon Pearl or Diamond version but in Pokemon Heart Gold and Soul Silver you can go to the Jotoh region.
walk
you can only go to kanto and johto
There Is A Pokemon Diamond Walkthrough If U Go To Pokemon .Marriland.com
go to canalave city and go to the Pokemon mart and talk to the other guy, if not there, try another town