same problem here...
1. created points in google earth - saved them as .kml
2. used GPSBabel to convert all .kml files into a single .gpx file
3. used humminbird pc to open .gpx file
4. receive error message: cvc-elt:1: cannot find the declaration of element 'gpx'.
I then tried to convert and open a single file instead of multiple - but i get the same error
So, I found another very simple converter (called KML2GPX) which gets it done... i can now open the .gpx file in humminbird pc. only problem is this simple converter only converts one file at a time. Not ideal when you have over a 200 hundred points you want to upload into your device
Any luck with fixing the GPSBabel - humminbird pc issue??