My guess:

(root level)/Library/Application Support/MakeMusic/Finale 27/Plug-ins

That was my first guess too. But, there is no plugins folder there! 

Please specify the steps you’ve already taken (to solve the problem), so that we can rule out things.

I suppose that the problem is this (please correct me if I am wrong):

You have downloaded a third party plug-in, and you are trying to put it in the correct folder - right?

If you launch Finale v27, and look in its Plug-ins menu, are there any plug-ins showing in the menu?

Also, take a look in

Preferences - Folders

What path is specified for Plug-ins?

That was my first guess too. But, there is no plugins folder there! 

Odd, there's a recent issue where, if you installed from the updater, it added an extra plug-ins folder.

You have downloaded a third party plug-in, and you are trying to put it in the correct folder - right?

Third party plugins on a Mac go elsewhere—and not always where one thinks. The standard Default is Library/Audio/Plug-Ins where you will see a number of folders. Components houses AUs, VST has VST2, VST3 is obvious. There may be more such as HAL for Steinberg, MAS for MOTU and others. In addition, some companies bundle their plug-ins within their application folders—Apple also does this nowadays. 

Which goes to Peter's request:

Please specify the steps you’ve already taken (to solve the problem), so that we can rule out things.

(root level)/Library/Application Support/MakeMusic/Finale 27/Plug-ins

should look like this:

while the Plug-ins folder should look like this: