ok, heres the scoop
first, use the search funtion. its a great and necessary tool
second, you know what miis are and how to make them right? well basically the friends codes are there so you can trade miis and send "mii-mail" back and forth to one another.
so for instance, lets say i add you to my wii. i have a bunch of mii characters and you dont really have any since you just made yourself and just purchased the wii.
i then add you in the addressbook, wait for you to add me and once that is set, i can send you my collection of miis. you can only do this if the person is in your address book (IE friends code)
so basically that is the only form of chat right now