Do all the following codes at the difficulty menu screen. Depends on how you 
config the keyboard. Press [f6] key for your current keyboard config. 
(eg. Zoom1 is default to "A" key) 

Infinite continue:
Up, Zoom1, Right, Zoom2, Down, Zoom3, Left, Zoom2, Up, Zoom1 

Stage Select:
Up, Up, Down, Down, Left, Right, Left, Right, Zoom1, Zoom2, Zoom3 

Invincible mode:
Turn Left, Turn Left, Turn Right, Turn Right, Up, Down, Left, Right 
(It should display "Invincible Mode" after you've type it. Please 
note the Turn Left key isnt the same as the Move Left key.
If you can't make it working, try config the Turn Left key to "Q" and 
Turn Right key to "E". Then type Q,Q,E,E,Up,Down,Left,Right should be okay)