To unlock each stage of level six, enter the following codes at the passwords screen

Password - Effect

MUVWNSZC Level 6 stage 1
UKRLNGHV Level 6 stage 2
UAGWBVPA Level 6 stage 3
UANWQEPE Level 6 stage 4
UANWBOPA Level 6 stage 5
MUBWNOET Level 6 stage 6
UKRLEBHT Level 6 stage 7
UANWQPPZ" Level 6 stage 8