Instructions
Select the "Settings" icon on your DSi dashboard, indicated by the wrench.
Navigate to the third page and select "Internet," followed by "Connection Settings."
Select "Advanced Settings," the last option listed on the screen.
Select one of the "None" options beside the listed connections.
Select "Search," and wait for the device to find your Wi-Fi signal.
Locate the SSID on your router if you have not changed it. Depending on your router, this location will vary, but it will be printed on the router itself.
Select the matching SSID on your DSi. A text field will appear where you must enter your WPA key.
Type in your WPA code, followed by "OK." This saves your changes, connecting you to the Internet.