It's not obvious even rooting through the manual so I'll leave this here for anyone else. To play Battle Isle 1 against the computer you need to enter the appropriate password on the main menu, a list of passwords is here:- http://www.gamefaqs.com/pc/585957-battle-isle/cheats

The game starts with a 2 player only map and you will not be able to select computer opponents unless you change this.

Thanks , your post just saved my day :) Downloaded it and was already worried that it´s not working properly.

if you want to Play 2 Player maps with single Player mode, you might overwrite the original single Player maps with the other files...

But do not expect heavy fights - the AI is too week für balanced maps!