Sorry: I had the impression that the standard configuration uses a laptop with its external (usually VGA) output configured for the projected image and with the laptop's internal screen still fully functional for interacting with the operating system and running programs. i.e. you'd select the script on the laptop's screen.
Unfortunately, there's no builtin Celestia hotkey for scripts that I'm aware of.
In principle, one could have a CelX Celestia startup script which provides one.
As I wrote previously, I'm not familiar with the Mac interface to Celestia. With the Windows version, if you're running full-screen, when you move the cursor to the top of the screen a menu bar appears. I don't know if the cursor is visible in your case. In Celestia v1.6.0, there's a scripts entry in the file menu which provides access to the scripts placed in Celestia\scripts. I don't know where the corresponding directory is in the Mac install, since Celestia can be installed in at least two different sections.
(It'd be really nice if some Mac expert would update the WikiBook to describe the Mac installation options at
http://en.wikibooks.org/wiki/Celestia/Installation )