The Mac file is named Celtx-2.7.dmg. This could be 2.8 or 3.0, by the time you read this book and get the file.

To install is a snap. Just double click on the .dmg file to mount the drive to your system. Then drag the Celtx icon to your Applications folder. Eject the mounted drive and you can delete the .dmg file. That's it!