First, you need to have a Pokémon game cartridge inserted into your Nintendo DS. Next, power on the DS and select the PokéWalker option from the main menu. After that, press the Register PokéWalker button. A screen will come up asking you to enter a name for your new PokéWalker. Once you've done that, press the OK button and your new PokéWalker will be registered!