At the mission selection screen, press Start. Select "Extras", "Enter Secret Code", then enter one of the following codes to activate the corresponding cheat function:

Master Control

Enter Cannonbolt, Heatblast, Wildvine, 4arms as a code. You can now go alien to alien with no timer.

Invincibility

Enter XLR8, Heatblast, Wildvine, 4Arms as a code.

All combos

Enter Cannonbolt, Heatblast, 4arms, Heatblast as a code to unlock all combos.

All locations on map

Enter Heatblast, XLR8, XLR8, Cannonbolt as a code to unlock all locations on the map.

Alien forms

Enter Wildvine, 4arms, Heatblast, Wildvine as a code to unlock all five alien forms.

Dark Heroes

Enter Cannonbolt, Cannonbolt, 4arms, Heatblast as a code. Your appearance will now be darker.

DNA Force

Enter Wildvine, Fourarms, Heatblast, Cannonbolt as a code. Your appearance will now be translucent.