(– by sibbles #2090 –)
OK, so that link downloads a .zip file, which is a compressed file. Download that one, double click it, and it will open into a new folder with a file that says something like darlinghydroline.gpx
You want the gpx file on your SD card, that then goes into the GPS unit.
If your computer can't unzip the zip file, I am assuming it's an older Windows machine. Download something called WinZip and you can use that to unzip as you need to. Google winzip to find that.