You must register a voter the first time you use the Everybody Votes Channel. To do so, simply choose a Mii that you've created on the Mii Channel. You can register up to six different Miis, but if there aren't any Miis in Mii Plaza on the Mii Channel, you won't be able to register any voters. Refer to the Wii Operations Manual (Channels and Settings) for more information on the Mii Channel.