In two player game, the first map code is FIRST (Default setting). In a game against the computer opponent, the first map code is CONRA.
From the manual: "2 - LEVEL/LEVELS 0-15 CONCERN MAPS WHICH ARE ONLY AVAILABLE IN 2 PLAYER GAMES NUMBERS 16-31 ARE MAPS USED WHEN YOU ARE PLAYING THE COMPUTER." This way you'd have to play through all the 16 two player maps first to reach the first single player level. This is most likely a bug in the game logic. Please refer for a complete list of level passwords to HOL or simply use 'conra' in the first field inside options.
