When at the main menu, choose the Stale Fish Tour. Then use the bottom option to input one of the passwords as below - these will bring you to the named stage against an AI player.

Effect	Password
Level 2 - Las Vegas	QPRH
Level 3 - Chernobyl	BKWV
Level 4 - Hanoi	GXJS
Level 5 - Baghdad	ZFRL
Level 6 - Panama City	KMTQ
Level 7 - Easter Island	LZBW
Level 8 - Atlantis	RSFN