a) Try to open LibreCAD -- it will fail, saying it's from an unknown developer, then:
b) Mac's System Preferences -> Security & Privacy
c) Under the "General" tab, you'll see "Allow apps downloaded from..." and it will mention "LibreCAD as recently blocked from user because it is not from an identified developer". Select "Open Anyway".
d) Try opening LibreCAD again -- you'll get a popup, still saying it's from an unknown developer, but now it will include an "open anyway" option -- choose that option and you're done.
(These steps are necessary for any downloaded software that isn't "signed" -- To "sign" software, Apple requires money (you need to be a licensed developer) which is why many open source projects don't.)
I can install libreCAD 2.1.3. The latest 2.2.0 will load, but never gets past the initial screen for me.