i think you need to let them know, via the boards or PM, so they add you.
I think then once both peoples code are entered on each others system, you can start messaging.
I dont remember getting a message form another Wii saying someone had added me, and i had to add them, we all just added each others codes and then were able to send messages.
Send me your code and i will add you.
Add me and you should be able to send me messages.