fldigi, flrig etc are distributed in 3 different disk image formats, for example:
Files may be downloaded from https://www.w1hkj.org/files, or https://sourceforge.net/projects/fldigi/files/
Development/test versions of fldigi can be found at https://www.w1hkj.org/alpha/fldigi
Open the dmg and install by dragging the launch icon to the Applications Folder.
Using an app that can’t be checked for malicious software might harm your Mac or compromise your privacy. fldigi and the other applications are signed and certified by Apple.
Configure the fldigi audio source on the fldigi configuration dialog:
enable the allow button.