Download FreeCAD dmg file "FreeCAD-0.20.0-OSX-i386.dmg" from following website.
Release FreeCAD 0.20 · FreeCAD/FreeCAD
Double click the downloaded dmg file and Drag&Drop the FreeCAD icon to application foloder.
macOS can only run applications authenticated by Apple inc. If you start FreeCAD, FreeCAD will be blocked. Then click cancel. Open "System Preferences" and select "Security & Privacy", then click "General". And click "Open Anyway" button at lower of "General" panel to grant an exception for FreeCAD.
If you do not want to change the System Preferences, you can also run FreeCAD by clicking the application icon with holding down the Control key and select "Open".