Type the following at the Password screen:

Password - Effect

KKCJDD - Level 1 Difficulty 1
KCCBBB - Level 1 Difficulty 2
KICICD - Level 1 Difficulty 3
KACAAB - Level 1 Difficulty 4
IKAJDC - Level 1 Difficulty 5
ICABBA - Level 1 Difficulty 6
IIAICC - Level 1 Difficulty 7
IAAAAA - Level 1 Difficulty 8
KKDNDL - Level 2 Difficulty 1
KCDFBJ - Level 2 Difficulty 2
KIDMCL - Level 2 Difficulty 3
KADEAJ - Level 2 Difficulty 4
IKBNDK - Level 2 Difficulty 5
ICBFBI - Level 2 Difficulty 6
IIBMCK - Level 2 Difficulty 7
IABEAI - Level 2 Difficulty 8
KOGJHD - Level 3 Difficulty 1
KGGBFB - Level 3 Difficulty 2
KMGIGD - Level 3 Difficulty 3
KEGAEB - Level 3 Difficulty 4
IOEJHC - Level 3 Difficulty 5
IGEBFA - Level 3 Difficulty 6
IMEIGC - Level 3 Difficulty 7
IEEAEA - Level 3 Difficulty 8
KOHNHL - Level 4 Difficulty 1
KGHFFJ - Level 4 Difficulty 2
KMHMGL - Level 4 Difficulty 3
KEHEEJ - Level 4 Difficulty 4
IOFNHK - Level 4 Difficulty 5
IGFFFI - Level 4 Difficulty 6
IMFMGK - Level 4 Difficulty 7
IEFEEI - Level 4 Difficulty 8