Trainers are third-party programs that run alongside the game to provide extra features, including level unlocking.
: Start any mission, then press the Print Screen (SysRq) button during gameplay. This will immediately complete the current mission and unlock the next one in the campaign.
This code works on all versions (Retail, GOG, Steam’s DOSBox version, and original CD release). It does not disable achievements (there are none) or affect gameplay. It only unlocks the mission select screen.