Link here to download Garmin Basecamp for Mac software directly from the Garmin Website - https://www8.garmin.com/support/download_details.jsp?id=4449
Mac® Requirements
Intel-based Mac running OS 10.13 or higher
1024 MB or RAM (2048 MB recommended)
If you have an older OS than above you can install an older version of Basecamp from here.
If you are having problems downloading Garmin BaseCamp for Mac from Garmins website you can also download directly from the Mac App Store - https://itunes.apple.com/gb/app/garmin-basecamp/id411052274?mt=12