When I try to install the Collaboration Plugin from the "Install Archi Plug-In..." menu option, a message box pops up telling me:
"Archi does not have write access to the plugins folder. Please quit and move the Archi app to a different location and try again".
I'm using Archi 4.4 on Mojave.
Just do that. Move Archi to a different folder and back again.
"Mac users on OS X 10.12 and later may encounter a problem due to Apple's recent implementation of translocation for unsigned apps (basically, unsigned apps like Archi are copied to a read-only area and executed from there, so the plugin can't be copied). We suggest that, before installing a plug-in, you move the Archi app to a different location (for example, move it from the "Applications" folder to "Desktop" or vice-versa.)"
I moved the Archi folder to Desktop, same result.
Maybe you not running with Admin privileges? If Archi can't write to its own internal "plugins" folder in the Archi.app then it hasn't been given write access.
Failing that, a manual installation would be to unzip the plugin zip file and move or copy the *.jar files into the "plugins" folder inside the Archi.app bundle (accessed by right-click "Show Package Contents").