Okay, here's what you have to do. Enter the "Rescue Professor Jones" level in "The Last Crusade," and go to the room with the rotating pictures and rugs. Once you get the box in the far right corner of the room out of the little hidden space move it to the upper level. Then PULL the box into the hole. With luck, you'll walk backwards RIGHT THROUGH THE WALL!!! Then you'll fall through the air. WARNING: don't do this unless you are prepared to lose some studs, (coins,) also, this only happened once for me so it might not happen for you.
At the 'Title' screen press Right, Up, are, L, X, Y, Right, Left, B, L, are, L, Down(2), Up, Y(2), X(2), B(2), Up(2), L, are, Start, Select. If you have entered the code correctly you will hear a confirmation sound.
Enter the following codes at the 'Title' screen to unlock the corresponding effect in the game. If you've entered the codes in correctly you will hear a confirmation noise. ('L' and 'R' in the code indicates the Left and Right shoulder buttons on the DS).
Unlock ALL Characters: At the 'Title' screen press Press X, Up, B, Down, Y, Left, Start, Right, R, R, L, R, R, Down, Down, Up, Y, Y, Y, Start, Select.
Begin the Game with 1,000,000 Studs: At the 'Title' screen press X, Y, B, B, Y, X, L, L, R, R, Up, Down, Left, Right, Start, Select.
Begin the Game with 3,000,000 Studs: At the 'Title' screen press Up, Up, B, Down, Down, X, Left, Left, Y, L, R, L, R, B, Y, X, Start, Select.