I purchased the Garmin 810 (bundle) from FMV.  I wanted to find a site to explain how to upload the GPX map files.  I found one.  I use an iMac & the directions are Windows based... so I know it works with OS X.  

An important note:  when the file uploads to your Garmin, go into the Garmin & into the "Courses" folder.  Rename the file (map) you've just uploaded.  Renaming it is very important because the GPX file (map) is a date of the year; probably the date it was created, but I don't know.

http://www.scarletfire.co.uk/transfer-gpx-file-to-garmin/

Be safe.