Before Miis can visit you, you must have at least one 'friend', so you must first share 'friends codes' .. or really the system codes. Both you, and the other person, must input each others codes before it works. (sucky, i know).
I explained how to find your code in a previous post (go to message board). Give this code to the other person (or post it here!). Now you have to input their code by adding it in the address book (also in the message board). Make sure they do the same!
The next two things you need to do is to set your Mii channel so your Miis can run around. From the Mii 'parade', you need to turn travelling on. Then if you want your own Miis to hang out, ensure that you have your Miis set to 'mingle', (edit the mii if not).
some links for help in case I was not clear. (I doubt I was.)
Sharewiis.com Faq
Miiplaza.net Faq
Nintendo's Mii Channel Faq
