At the Title Screen, press B, A, Left, Down, Up, Right. You will be presented with a Stage Select as well as options to start with all weapons and have infinite health.
Level Passwords
Password - Level
1C6YPT - Desert 2
0RK01D - Cave
6PC49G - Jungle 1
JMF69J - Jungle 2
0F1BR8 - Medical Center
LY7VRQ - Ape City 1
2TMQ1V - Ape City 2
V!TG14 - Ape City 3
DTRZ1L - Beach
XZPFR0 - Mountains
06H91V - Underground City 1
G!WN9Z - Underground City 2