You will need to set your router for wep security or remove security on your router to use the nintendo ds at this access point. Wep is the only security that is compatible with the nintendo ds wi fi connection.

Belkin Official Support How To Connect Your Nintendo 3ds
What is a wep key for 3ds. If your wireless network password does not meet these requirements it is not a standard wep key. The nintendo 2ds and new nintendo 3ds systems support the same but. The nintendo 3ds and nintendo 3ds xl support wep wpa psk tkip wpa psk aes wpa2 psk aes and wpa2 psk tkip security. When wep or wpa is turned on enabled your router uses a special security key combination to allow only devices that know this key to connect to your wireless network. If you wish to connect a nintendo ds or nintendo ds lite to your network this key will need to be. The 3ds itself is compatible with wpa security when its in 3ds mode but when running a ds game in ds mode its not able to connect to a router with wpa security.
A wep key is a security passcode for wi fi devices. Wep is the only security that is compatible with the nintendo ds wi fi connection. Wep key is a security setting for your router. You will need to set your router for wep security or remove security on your router to use the nintendo ds at this access point. Wep keys enable devices on a local network to exchange encrypted mathematically encoded messages with each other while hiding the contents of the messages from easy viewing by outsiders. How wep keys work.
Wep stands for wired equivalent privacy a wi fi wireless network security standard. Wep is the only security that is compatible with the nintendo ds wi fi connection. If you know you do not have security on your router you may leave this field blank. Some of the nintendo support pages indicate the device only supports wep encryption versus wpa2 or wpa. Wep key is a security setting for your router. Once you know the wep key you will need to enter it when prompted.
The wep key is usually found in the security tab of your wireless router settings. Wep key is a security setting for your router.