[cnojek]

How can I transfer waypoints and routes from MacENC to a handheld Garmin GPSmap 78s?

[GPSNavX]

I believe the Garmin 78s can accept GPX files. So you should be able to use the "Transfer To" "GPX File.." on the "Waypoints" or "Routes" menu to save a GPX file. Connect the GPSMap78 via the USB cable, then copy over the file to the GPSMap78 drive. It may have to be put in a specific folder. Refer to your Garmin manual on the procedure.