After downloading the KnotPlotDiskImage.dmg.gz file, your computer may or may not unzip the file. Assuming that it does, it will probably leave a KnotPlotDiskImage.dmg file on your desktop. Double click on the KnotPlotDiskImage.dmg file and it should mount a KnotPlot disk onto your desktop. Look inside the KnotPlot disk and you'll find the KnotPlot application and a README file, and possibly other stuff.
Copy the KnotPlot application to a convenient location. You can now eject the KnotPlot disk and delete the KnotPlotDiskImage.dmg and KnotPlotDiskImage.dmg.gz files.
Please let me know if these installation instructions don't work for you.
Rob Scharein