Only on the wiiu you can dm, for friending check their name in their profile and add them to your friend list, also if you want to dm you need to be a friend of the person... I don't know if that works on 3ds because I don't have that... cheers!

on 3ds, go to friends list. go to your profile and check the friend code on the top screen. you have to exchange it with someone else. hope this helps :)