My pal, using the Windows Garmin POI Loader has a choice, in the first window, of Garmin Device - MapSource (Advanced) - Custom Folder (Advanced) and, in the next window of Remove POI's or Add POI's.
This Mac version has offers no such choices. Once it finds your device it offers only one choice: add POI's.
Come on Garmin! We paid as much for our GPS units as did the non-Mac using purchasers. How hard would it be to offer comparable software for Mac users???