Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "shellcfg.h" file in the "darkshell" directory. Change the "#define BTN_MISSION_COEFFICIENT 150" line to "#define BTN_MISSION_COEFFICIENT 157". The new "Cheat" icon in the lower left corner of the screen will allow any mission to be selected on the dial.
Bonus level
Click on the spiral on the level selection screen after completing level twelve to play the Death Blow level.