You *WILL* be spoiled if you read any further. I take no
responsiblity for the disappointment you will feel if you
inadvertantly read this information.
2. How to use the MSG
The MSG is divided into sections, one for each chapter of the game, with each level in it's appropriate chapter, as well as some general information sections. To find information on a certain area of the game, go to the appropriate chapter and use "Find" in the File menu.
In case you don't know how to determine the name of the level,
just switch over to map mode (default key: 'm') while playing Marathon, and look at the top of the map. The name of the level will be there.