To play as Luigi in Single Player mode, at the "Select A File" screen, highlight a file, then hold L + R and press A.
To play in Secret Challenge mode, complete the game, then start the cleared game file, pause the game while at the world map and press L, R, L, R, X(2), Y(2).
Unlock the following Worlds:
World 4: defeat the Boss in the last castle of World 2 with Mini Mario
World 7: defeat the Boss in the last castle of World 5 with Mini Mario
World 8: defeat the Boss in the last castle of World 6 with Mega Mario