Nyquist plug-ins are just plain text files, with a file name extension of “.ny” (rather than “.txt”)
In current versions of Audacity, after moving the plug-in file into Audacity’s plug-in folder, you need to enable the effect in the plug-in manager: Plugin Manager - Add / Remove Effects, Generators and Analyzers - Audacity Manual
If the plug-in does not appear in the plug-in manager, then you’ve probably not put the plug-in file into the correct folder.
The correct folder is:
~/Library/Application Support/audacity/Plug-Ins
Note the squiggle (tilde) at the beginning. “~/” means your home folder. Use “Go” (main Mac Desktop menu bar) and copy/paste the address to open it in Finder.