When I try to download the map to my Mac I see a .gmapi folder instead of a .gmapi file

If you downloaded Garmin Basecamp from the App Store you are missing Garmin MapInstall and MapManager software necessary to download and view the map. You can download MapInstall and MapManager here or you can reinstall Basecamp from the Garmin website. The download from the website installs Basecamp, MapInstall, and MapManager at the same time whereas the download from the App Store only installs Basecamp.

Once you have Basecamp, MapInstall, and MapManager on your computer you will see a .gmapi file instead of a folder when you download the map and you should be able to continue with the installation process.