Instructions
Load "Pokemon SoulSilver" or "Pokemon HeartGold" to the main menu.
Select "Connect to Pokewalker." Answer the questions as normal until given the option to "go on a stroll."
Press the "L," "X" and "Down" buttons simultaneously. Release the buttons to receive a warning about deleting all records from your Pokewalker.
Confirm the data deletion and continue through the questions until you receive a confirmation that you reset your Pokewalker.