I've gotten friend vouchers from one of my friends before, and it's simple really.
All you have to do is put a friends Wii mac address, (which can be found in the Wii message board) into your Wii address book, (which is also in the Wii message board), and have your friend put your mac address into their Wii address book.
Once this is done you have to wait a bit while they're registering and then you can send messages via "Wii mail" to your friend.
To send and recieve friend vouchers though requires both of you to have Metroid prime 3 save data on your Wiis, just start up Metroid 3 and go to the friend voucher menu in the "extras" section, click on your friends icon and send a voucher.
It's as simple as that