Each trip with a Pokemon in the pokewalker gains it one level as soon as you return it to the game. It doesn't matter how many days the Pokemon is in the walker, it will only gain one level upon its return.
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Get a combee from the pokewalker and level it up so it evolves.
You can find Carvanha with the PokeWalker on the Warm Beach path. Just level it up from there.
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.