Enter these on the passcode screen:

TUSKFISH - Unlock All Missions 
BANNER - Invincibility 
WRASSE - Bullet Shield 
JAWFISH - Unlimited Ammo 
BATFISH - Silver Bullet 
CARDINAL - Achilles Head 
BOXFISH - All Replay Items 
PARROT - Invisible Soldiers 
DOTTYBACK - Men With Hats 
BETTA - Rubber Grenades

Control main menu:
Move the Right Analog-stick to control the background at the main menu.

Search for Yomashita Gold: Entrenchment tool:
During the mission, you will find a Buddha statue sitting Indian-style holding a 
lotus flower. Approach it and you will take it. Near the end of the mission, after 
you save the pilot, go to the end of the hall and place the lotus in the fountain in 
front of the Buddha lying down. A message stating "You have achieved 
enlightenment" will appear. The fountain will lower and a door will open to your 
right, Indiana Jones style with a great earthquake. Take the shovel inside and a 
message stating "You have the entrenching tool" will appear. You can now load 
and switch your weapons much faster for the remainder of the game.

Day Of Infamy: Unlimited ammo cheat: 
Note: This trick requires the entrenching tool from the Search For Yamashita's 
Gold mission. As you are making your way up through the ship you will see a 
soldier that has been sprayed in the face with steam from an exploded pipe. 
Approach the soldier and press A. The message "Entrenchment Tool Used" will 
appear. This will unlock the "Unlimited Ammo" cheat in the bonus menu. 

Fight For The Philippines: Bullet shield cheat:
Note: This trick requires the entrenching tool from the Search For Yamashita's 
Gold mission. Once you reach the baseball field, in one of the corners is a pile of 
dirt. Press A by it and you will unlock the "Bullet Shield" cheat in the bonus menu.