After you have un-zipped the FFmpeg archive, the best place to put it is:
This is the same folder where the LAME installer puts the LAME files. You must put all the FFmpeg files there, not just libavcodec.55.dylib.
Note that this is not your user Library folder, but the system Library folder. You can get to this folder using the menu Go > Go to Folder, or by opening a Finder window on your startup disk, then opening the “Library” folder you see there. Once you have the FFmpeg files in that folder, Audacity should find them automatically.
I’m using Mac OS High Sierra 10.13.6 and Audacity 2.1.0
I’ve tried to download ffmpeg libraries in various ways, following all available instructions on the Audacity pages, but my Audacity still doesn’t find them. I’ve even tried the “Alternative zip download for expert users”, but rather than the three files I was told to expect, I just got a single file called “libmp3lame64bit.dylib” which I can only open on terminal. I’ve been trying to sort this out for quite a while. Any help would be appreciated!