If you are going to manually place the file on the 810, then you have to put it in the /garmin/newfiles folder. After restarting your 810, it should import the file. If it does not, then the file wasn't acceptable. Unfortunately your edge doesn't tell you what it didn't like about it. gpx files are not always a course that the 810 will import. Sometimes it's simple things in the header records of the file that it's not finding.

Are you certain the file is a .gpx track file? Try a .tcx course file instead.

If you can't browse the file structure of the 810 with your tablet, then likely you don't have the drivers and app to do so.