I purchased the download version of BIAB Pro for Mac 2019. I had some installation issues, at first, too.

It turned out that the installation was incomplete. The Installer put some packages in my Root folder that needed to be installed. The original disk images unpacked those packages but never executed them. Once I ran those packages and installed the missing files, all was well. Maybe you're having the same issue?

Somewhere I read that some packages would be installed as Devices. When they didn't appear in Devices, that's when I dug deeper and found them in the HD's Root folder.

Maybe that will be of some help to you, I hope.

From Page 10, Chapter 1, 2019 Band-in-a-Box Mac Manual.

"Band-in-a-Box Folder
The /Band-in-a-Box folder has a clean look with three application files - Band-in-a-Box, Band-in-a-Box-32bit, and BBM4 (used for the Band-in-a-Box DAW Plugin). The rest of the files are in subfolders.
The folders include the Drums folder for RealDrums files and the RealTracks folder for RealTracks files. If any of these folders is moved or the files are installed in a custom location, the new location must be entered in the RealDrums Settings dialog or the RealTracks Settings dialog so that Band-in-a-Box knows where to find them.
The Preferences you set in the program are saved in the Preferences folder, and this folder also has a file named Intrface.BBM with all the program settings including MIDI and audio driver setup. If this file is removed to the trash the program will open like a new installation the next time it runs. It will prompt for all initial setup selections. This is useful as a “global reset” if there are system settings that have been changed by accident."

There's your answer to BBM4 app.