as far as i know, just like any other version of sauerbraten, the "mac-version" also has an "autoexec". If not, just make one by using notepad (and change the extension from ".txt" to ".cfg" and put it somewhere close to "config.cfg" (so in the same folder)...

...or name the textfile mapname.cfg and put it in the same folder as mapname.ogz, that is: /packages/base/ (of course replacing "mapname" with the actual name of your map).

The Mac-equivalent of Notepad is Textedit.

What is the script supposed to do and when?

See my comment I just posted elsewhere:
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