I am new to MacBooks. I have a Macbook Pro with Basecamp 4.4.6. I also have an etrex Vista HCx and am new to installing maps on the device. I have been able to load Openfiets Lite onto the device successfully. I note that the Edit mode on Basecamp is inhibited i.e. the Delete and Delete from etrex functions, are greyed out and not functioning! Any ideas?
Where did you get BaseCamp? If you installed it using the Garmin BaseCamp installer, then in Applications you'll find two other Garmin apps that will help, MapManager and MapInstall. MapManager deals with maps on your computer, and I think it's the app that starts if you double click on a gmap file like the ones you downloaded from OpenFietsMap. MapInstall is what you use to install maps on a device. If you got BaseCamp from the App Store, then you should go to http://garmin.com/osx and get MapInstall and see if that helps. A separate MapManager installer isn't available from there, but you can always download the BaseCamp installer (just ignore the fact that it's "available in the Mac App Store") and get it that way, or maybe the MapInstall installer also installs MapManager.
Just to make sure, did you select any map tiles before telling MapInstall to send them to the device? It's been a while since I last used MapInstall, but I seem to remember that you have to select something (or use command-A to select everything) in order to actually get it to transfer to the device.
You may need to zoom in to see them. Also ensure you don't have other maps selected.