See this topic for installing Audacity on a Mac: https://forum.audacityteam.org/t/installation-do-not-run-audacity-from-the-dmg/31711/1
Then, before installing any plug-ins, check that Audacity is working correctly with no error messages.
Once that part is correct, installing plug-ins should be straightforward, but you will need to say which plug-ins you want to install because installation varies depending on the type of plug-in. Instructions for installing “Nyquist plug-ins” are here: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#Installing_Plug-ins