I currently have this question into the Chat site and am waiting for their response. Will post back here if given one.

I've successfully installed the PlusPAK upgrade and 49-PAK bonus, to my iMac for the Mac 2018 version of BiaB (Mac OS High Sierra); however using the same installer files (or even a newly re-downloaded installer file) I can't get any installers to open/run on my MacBook Pro - which is running only Mac Sierra OS.

Any suggestions why the MacBook Pro does absolutely nothing upon double-clicking the installer file (or even right-clicking, 'Open' it)? I had no issues at all running all 11 (9 files, plus 2 49-Pak bonus files) on the iMac

SOLVED: on Mac OS Sierra (not High Sierra), the System Preferences' "Gatekeeper" no longer allows opening apps from "Anywhere" (only apps from Mac App Store and identified developers).

I tried control-clicking to open the app - - as one online suggestion recommended - - to no avail.

Finally using the Terminal command line at this link allowed me to open the MacBook Pro to downloading from "Anywhere"
http://osxdaily.com/2016/09/27/allow-apps-from-anywhere-macos-gatekeeper/

Hi DH00,

I'm glad you got it to open, but you really shouldn't have had to open Terminal to do this. I would have expected right-clicking and selecting Open to at least give you the option of bypassing GateKeeper. Were you logged in with an admin account?

Were you double-clicking on the DMG files, or the PKG files they contain?

Thanks
Kent
PG Music