I am answering my own question -- this is the procedure that worked for me:
Procedure to load maps from GPSFileDepot on to Garmin via Mac
1. connect Garmin device via USB cable to Mac
2. Download selected file (*.tgz) from GPSFileDepot
3. Open with "Stuffit Expander" which should create a *.gmapi file
4. "double click" *.gmapi file
5. Open Garmin MapInstall App (file should be present)
6. "click on" Send Maps
Procedure to load maps from GPSFileDepot on to Garmin via Mac
1. connect Garmin device via USB cable to Mac
2. Download selected file (*.tgz) from GPSFileDepot
3. Open with "Stuffit Expander" which should create a *.gmapi file
4. "double click" *.gmapi file
5. Open Garmin MapInstall App (file should be present)
6. "click on" Send Maps