Free Play Mode
1) Go to the main menu at the title screen
2) Choose Adjust Settings
3) On the settings menu, choose Miscellaneous
4) On the miscellaneous menu, move to credit limit and press Square, O, Triangle, and X at the same time and then let go. The credit number will change to FREE PLAY.
Mission Select
1) Go into settings menu
2) Go into difficulty menu
3) Hold down R1, R2, L1, L2 and press START
16 Credits
1) Using the 2nd controller, set your credit limit to 9 and Start your game
2) When the ship is launching off, hit start on Controller 1
3) Let the 2nd Controller ship blow up and then pause the game to see your newly acquired credits.