First off, I'm not claiming I know how to use this thing. In fact, I have little-to-no clue how it works. Here's what I did: I downloaded Aleph One, then the Marathon thing that goes along with it, and then stuck the Marathon directory inside the Aleph One directory, then launch. This is probably totally wrong, but I couldn't find anywhere in the documentation about how to install or troubleshoot it. Here's string of Console messages:

Anyways, all help is appreciated!

Kind of backwards. Anyways, from the top, go to this page:

http://source.bungie.org/get/

Get whatever scenarios you want, download the version of AlephOne that your OS needs, then take the AlephOne app and drag (or copy and paste) it into the scenario folder you want to play.

Edit: To expand a bit, take the scenario.zip file and decompress it to wherever you want it to go. Then take the Aleph One.zip file, decompress it, and then drag the AlephOne application and drop it into the top level of whatever scenario you want to play. Thus, you can put copies of the app into every single different scenario you have and you don't have to worry about moving the app around all over the place. I don't think that you need the Aleph One folder after that, really, but it does have a troubleshooting guide in it.

Awesome! I got it to work, thanks for the help!
...does it do network games?
GutlessWonder++

It does do network games. Everyone who does play online usually do so on Marathon Infinity.

Aleph One has built-in support for Mariusnet. You can either log in as a guest or create an account and enter the username/password in Preferences > Player if you'd like a custom screen name.

If you want to host games you'll need to forward ports 4226 and 15367.

Sweet. Another question: I played the original Marathon in another folder with another copy of Aleph One, and now the Marathon Infinity screens (except for the main screen) are the old-school font style instead of the sleek Infinity style. Does anybody know why this happens / how to fix it?

Aleph One now uses themes, which are generally associated with specific scenarios. Change your theme in the environment preferences. Theme support keeps, changing, though, so I don't know offhand where to find it.

A little tidbit of info.

For more and better help, go to The Pfhorums, or Marathon's Story Forum. You'll get everything you need to know about Aleph One, Marathon, and all of the scenarios, right there.