Celesita's installer does define some registry entries under Windows. They're used to associate Cel://URLs and scripts with that copy of Celestia. I'm guessing something equivalent may be happening in the Mac installer.
Please distribute your Addons as Zip files. exe files only work on Windows systems. Celestia is cross-platform: it runs under many different types of operating systems. Many schools have Macs. Some probably have Linux systems now.