Pause the game, and enter the following codes:
Code - Effect
X, Down, Down, Up, Left, Right - Faster Build-Up
X, Left, Right, Up, Left - Get $10,000 Instantly
X, Right, Right, Left, Up - Get 100 Population Instantly
X, Up, Up, Left, Right, Left - Get access to all missions in Replay Mode
Right, Left, Right, Right, and X - Invincible Units
Hold all 4 shoulder buttons then press X, Up, Up, Left, Right, Left - Level Skip
Level Skip Note:
Repeat this code until the confirmation message appears. Then quit your game and when you get debriefed, press X to advance to the next mission.