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