If you go into the Extras section, you can share your MP3 information with members you added into your Wii's address book. In addition, you can send something called a Friend Voucher, which can be turned into a Friend Credit by sending it to someone in your Wii address book that has MP3 as well. You use Friend Credits, in addition to other credits, to purchase stuff from the various Galleries.
The WiiConnect24 option is Off by default, so you need to turn it On in order to receive Friend Vouchers.
I just turned mine on, so send vouchers my way!