Instructions
Turn on your Nintendo DSi.
Open the DSi's "System Settings" menu. This menu is marked by a small gray "Wrench" icon on the console's main menu system.
Select "Internet." Tap the name of the Internet connection to which you are trying to set up.
Select the "Change Security Settings" option. Select the type of security settings that are present on the wireless connection. For example, if the wireless connection has WPA encryption, select WPA. If it has WEP encryption, select WEP.
Type your wireless connection's password into your Nintendo DSi. Select the "Save" options. The DSi will remember the specified security settings and will use them whenever the network in question is in range for connection purposes.