As soon as you appear on level two (don't move the walker at all), type "EAT LEAD MUDDY FUNSTER".
On the title screen (the one where it says press fire to start) simply type in "WALKER" and a red line will appear in the center of the screen. Now during play, try any of these cheat key combinations (you may have to hold them for 5 seconds):
'Help' and 'I' - invinciblity
'Help' and 'L' - level skip
'Help' and 'R' - restart level
'Help' and 'K' - ?