To unlock these cheats you need to enter the button combination indicated during gameplay in The Fortess Of Solitude

Left, up, Right, Down, Y, X, Y, Up, Right, X - All Unlockables
Up, Up, Down, Down, Left, Right, Left, Right, Y, X - God Mode
Y, Right, Y, Right, Up, Left, Right, Y - Infinite City Health
Left, Y, Right, X, Down, Y, Up, Down, X, Y, X - Unlock All Powerups
Up, Right, Down, Right, Up, Left, Down, Right, Up - Unlock Bizarro