Instructions
Start your "Pokemon SoulSilver" game, and turn on your Pokewalker.
Select "Connect to Pokewalker" from the main menu of your "Pokemon SoulSilver" game.
Hold down the "X," "L," and "Down" buttons on the Nintendo DS.
Press "Yes" when the game asks you if you're sure that you want to delete the Pokewalker data. Wait a few seconds and the Pokewalker will restart, completely reset.