Nintendo puts the security settings on the game card also (where it can't be updated, arrgh). Because WW is a DS game and DS only had WEP security, it requires WEP to connect. WEP is outdated so your 3DS and router are probably using WPA.
in order to connect WW you need to switch all your security settings to WEP. WEP is not very secure so you have a choice. I am going to swap back to WEP but it really bugs me. I have 9 devices that will need to be set back to this old security standard in order to use WW!
I just bought WW because I couldn't waiting for AC 3DS was driving me batty. I never played WW so I need the fruit trees from a friend.