man bash will tell you much, much more than you'd ever want to know about shell environment variables (which __GL_FSAA_MODE is one of), but it's probably going to make almost no sense, so I'll just tell you what to do.
First off, you have to be starting these games from a terminal.
Now, before you actually type in the command to start the game, type in this:
Substitute whichever setting you want (they should be documented in the readme) for the 2 there. After you've done that, then you start up the game normally (from the same terminal), and it should see the environment variable.
BTW, the export tells the shell to not only set this variable for the current shell, but any of its children (if your game is started by a script, you will need this; if not, it's still a good idea). The next part (up to the equals sign) is the variable's name, and the stuff after the equals sign until the next bit of whitespace (either return or space or tab) is what to set the the variable to.