I’m using flrig/fldigi (4.1.09) on my Mac laptop OS X 10.14.6. I had trouble with the Waterfall not working/inop. Completely black.
Here’s why: During the fldigi install process, I apparently did not “allow incoming" to the ports when prompted. Just a habit...
To correct this, open System Preferences in the Dock.
Select the “Security and Privacy” pane.
Select the “Privacy” tab.
Then, in the left window, click on “Microphone” and in the right window under “Allow the apps below to access your microphone,” make sure fldigi is checked. (You should not need to do a restart, but it never hurts).
This will allow the signal from your interface unit (Yaesu SCU-17 in my case) to be displayed on the waterfall. If needed, you can start with the values: -30 and 70 settings next to the” WF" tab in the lower left corner of the fldigi screen.
Also, always make sure to, in order: 1) Radio-ON with USB cable plugged into your Mac from your interface 3) Launch flrig (if that’s how you control fldigi) and then fldigi.
Now you should Open System Preferences again and select the “Sound” panel. Make sure "USB Audio CODEC" is selected in both the Input and Output tabs.
Hope this helps someone. I’m just figuring out all this as I go…