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.
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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.
Trapinch can be found on route 228 on both Diamond and Pearl using the PokeRadar.
Use the pokeradar. you have to put the game in your cra[
You can't catch whismur in Pokemon pearl or diamond but but you can catch its evolved form loudred with the pokeradar on the top of mt coronet.
Use the Pokeradar on Route 228, or transfer from RSE