Unfortunately, the only way that the game is guaranteed to work is if you already own Marathon 2 for the Macintosh.  There is no reason to expect that the PC version will not work if you own a copy of Marathon 2 for the PC, however it has not been tested on this platform.  Unfortunately, the levels do not appear to work with the demo versions of Marathon 2 for either platform.  This is part of the reason why a full web walkthrough of the game is included on the website.

Gameplay:

The game plays essentially the same as Marathon 2 normally would play, with the following exceptions.

Terminals:  Terminals are key to moving the plot forward. After you've done something important (broken into the prison, for instance) the contents of terminals may change.  The locations of terminals may also imply that you're doing a certain action.  For instance, activating the terminal over the bed in level 1 means that you've decided to go to sleep and ignore your friend's plight.

Weapons:  You don't have any at the start.  And you're not well equipped to survive a firefight.  You also have a low tolerance for damage.  You can't even take one shot from the Blues, while they can take many punches from your fists.  So don't fight.  You'll lose.
    If you do find another weapon, it will come with instructions. To bring up your fists, use the next-weapon key (on my favorite setup, it's the + key).  Then punch with the trigger button and punch with the other fist by hitting the second trigger button.

Jumping:  There isn't any in this game.  But if you need to leap out into the air, it is recommended that you run as fast as you can. Hitting the ground from a high altitude isn't pleasant in Brightland, even though it's harmless in the original Marathon.

Save Terminals: They look like terminals, kind of, but when you activate them, they save your game.  Nothing especially different between these and Marathon's, but be careful when you save your game; you may have already made a choice that has doomed you to a very short life.  Wouldn't want to save that choice, would we?  It's easiest just to use different file names to save.

Getting out of the house:  Some people have trouble getting out of the house in Monochromia.  The reason is that the door is locked until you read your journal.  It was made that way on purpose.  Just stand in your chair for a while and the game will unlock the front door.  It opens a bit slowly but is otherwise normal.

Difficulty:  It's not easy.  Even on the level marked "easy" it's quite difficult.  You've been warned.  Realistically it should be difficult to break into an enemy prison.  But it can be beaten without cheating.  It has been done.
    If worse comes to worse, the walkthrough will give you a clue as to how to win.
    Of course, there's more than one way to "win".  If winning just means not dying.  Saying any more would be giving too much away.  Enjoy.