Instructions
Turn on your Nintendo DSi and open the "Settings" icon from the main menu.
Select "Internet."
Choose "Connection Settings." Tap the "None" icon to load a new connection configuration file.
Select "Search for an Access Point." Tap the name of your wireless network and type in the security password (if required). Select "Save Settings" to save the connection. The first of the two DSi consoles is connected to the wireless network.
Repeat "Step 1" through "Step 4" for the second Nintendo DSi.
Insert one copy of the multiplayer game you want to play into each Nintendo DSi. Note that if your game supports DS Download Play you only need one copy in one DSi. Not all games support DS Download Play -- those that do, print this information on the back of the game's case. Select "Multiplayer" from the main menu of the game to play a multiplayer game using the two Nintendo DSi consoles.