Hi, I have downloaded the manual for this device. Can you try the following please and let me know how you get on.
Disconnect the USB cable from your mac.
Turn off your garmin
Plug the USB cable directly into a USB port in the mac and then into the garmin.
The device should automatically turn on and go into MTP mode or USB mass storage mode. A picture of the device connected to the computer should show up on the Garmin screen.