The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
This is an f-stop timing calculator to be used in a photographic darkroom. 'Way Beyond Monochrome' by Ralph W. Lambrecht and Chris Woodhouse gives a brilliant introduction into f-stop printing. Some chapters are available for free from waybeyondmonochrome.com