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.