You must first collect all the pieces for each code.

999 mode
Uber difficulty, 999 health, unlimited ammunition.
1: 02 03 04 05 06 07 08 09
8: 07 06 05 04 03 02 01 09

Hardcore mode
Uber diffculty, all health and armor drops disabled.
3: 06 09 03 06 09 03 06 09
6: 03 09 06 03 09 06 03 09

Ironman mode
Uber difficulty, one life.
4: 08 03 07 02 06 01 05 09
5: 01 06 02 07 03 08 04 09

Walk In The Park mode
I Am Death Incarnate difficulty, HUD disabled.
2: 04 06 08 01 03 05 07 09
7: 05 03 01 08 06 04 02 09