To download it you can't press download, You have to right click and choose "Save as..." Then leave it and save it.
To install it after downloaded (Mac): Go to finder >> Downloads >> you will see diagscanline3.zip. Leave that window and open a second finder window, go to "Library" >> "Application Support" >> Gimp. On the 1st finder window, drag diagscanline3.zip to the "Patterns" folder in the 2nd window. Open gimp (If already open quit X11/Gimp and then re-open). It worked for me!