Unlock Hidden Ranking:
Get 100,000 kills to become an 'Addict'.
Unlock Hidden Characters:
Press the 'View' button 5 times and then use the joystick to go left or right to make your selection.
Unlock Level Select in Deathmatch:
When you win in Deathmatch hold down the 'Jump' and 'Drop Weapon' buttons to bring up the 'Level select' screen. Then use the right joystick to go up and down to make your selection.