One way of doing it on the Mac, if you are using Safari, is to click on the GPX file button under the Download section of the cache webpage. This will automatically put the file in your Downloads folder which normally pops up on the screen somewhere. It'll be either the first or the last in the list but it should be viewable in the list - look for the matching GC?????.gpx name (? represents any character). Click on the magnifying glass icon to the right; this opens up the Downloads folder in another window. Close the first Downloads windows leaving the new one open. Then, double click on the GARMIN icon on your desktop. Now you should have open, Safari, one Downloads windows and the Garmin window.
Make sure you can see the GC?????.gcx in the Downloads windows and the GPX folder in the Garmin window. When you can, simply drag and drop the GC????.gcx file onto the GPX folder. You should see the new gpx file in the Garmin window now. If you can, you have successfully transferred the cache to your device.
