You get on Pokemon Soulsilver and choose the option Connect to Pokewalker. Then, you choose Return from Scroll.
To get rid of items you've acquired or Pokemon you've caught you choose Receive Gifts.
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You can't put a Pokemon in the pokewalker in Pokemon platinum, as the pokewalker was designed for only Pokemon Heartgold & Soulsilver.
By unlocking the pokewalker route
You do not need a Pokewalker to trade Pokemon. The DS has built-in wireless communications. The DS does not need the Pokewalker to trade Pokemon.
If you reset your pokewalker your Pokemon will go and all your data will be deleted. That includes the steps places and watts. DONT BOTHER DOING IT UNLESS YOUR HAVE STARTED A NEW SAVE AND THERE IS A POKEMON ON THE POKEWALKER WITH A POKEMON ON IT!!!
NO The Pokewalker was made especially and only for SoulSilver and HeartGold, so the only way you could walk a Pokemon from your Platinum game is if you traded that Pokemon to SoulSilver\HeartGold and then transfer it onto the Pokewalker