Instructions
Launch "New Super Mario Bros." on the DS or DSi with the copy of the game.
Tap "Mario vs Luigi."
Tap the second option. Your Nintendo DS or DSi will wait until a signal is detected.
Turn on the Nintendo DSi that does not have a copy of "New Super Mario Bros."
Tap the "DS Download Play" icon.
Tap "New Super Mario Bros."
Tap "Yes." A connection will be established and the multiplayer portion of "New Super Mario Bros." is downloaded onto your Nintendo DSi, at which point the game will begin.