Dave:
Stu's calculator will not run by itself, it is a worksheet that runs inside a spreadsheet program.
You need either Excel or Open Office (Calc) to run the program. Open Office is free from Sun Micro Systems, but a large download. The .PDF file is the instructions that can be opened with Adobe. Save the file (to desktop or in your "my documents"), then either double click it to run it (assuming you have a spreadsheet program), or start the spreadsheet program, and then use "file" - "open" and pick the file you downloaded to open.
BobW