ThatObviousUser
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It doesn't say how
It's specific for C-Gear skins but it'll work for Arceus too:
Andrex said:The C-Gear also features customizable skins. These skins may be unlocked using a password then downloaded from the Pokémon Global Link. After connecting your Pokémon Black or White game card to the Global Link, you can go to the Promotions page on the Global Link website. There, you will be able to redeem the passwords you find for the C-Gear skins. After redeeming them, head to the Customize page and select the appropriate C-Gear skin, Pokédex skin, and Musical Show background. Then on the C-Gear in your Pokémon Black or White game, select “Online” and then “Game Sync.” Your new skin and settings should be transferred right over!
If I refresh the OT3 I'll make the steps easier. Of course, you'll have to meet and "battle" Arceus in the Entralink Forest.