Download and installation
You can download the program by clicking one of the download links.
Download: AutumnLeaves.dmg.tgz
Download 2 (www.xs4all.nl)
Download 3 (www.euronet.nl)

This should open "Stuffit Expander", resulting in a white disk image named "AutumnLeaves for Mac OS X" on your desktop:

A window opens (if not, then double-click the white disk icon) and you will see the application icon:

Installation
You can drag the icon to your Applications folder, or put it anywhere you like, for example on your desktop, or to the Dock.