The Pokéradar is a Pokémon-finding device that lets you find certain Pokémon you cant' see in grass without using the Pokéradar. For example, you can get larvitar, Ditto, and other Pokémon using the Pokéradar while in tall grass. You get the Pokéradar by completing your Sinnoh Pokédex. To do that you have to have seen all 150 Pokémon in the Sinnoh Dex, which is different than the GBA games because on those you have to catch 'em all. On Diamond/Pearl you only have to see them. If you're having a problem seeing a Dialga on your Pearl version, go to Celestic town and talk to an old woman in her house and she'll show u a picture of Dialga (which the Pokédex counts as seeing). I'm pretty sure she only shows you the picture of Dialga after you encounter Palkia after defeating Team Galactic. Good luck!
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Trapinch can be found on route 228 on both Diamond and Pearl using the PokeRadar.
The Pokeradar is not an option in Pokemon SoulSilver. The Pokeradar is only available on the Pokewalker accessory that came with your copy of Pokemon SoulSilver. Plus, the Pokeradar was introduced in Pokemon Diamond, Pearl, and Platinum, not in Gold, Silver, or Crystal.
On route 228 use a pokeradar at the top.
Use the Pokeradar on Route 228, or transfer from RSE
After seeing all the Pokemon in the Sinnoh Dex go talk to Professor Rowan and Professor Oak will come and give you the pokeradar as well as the National Dex.