Notice that the Modeler will search for plugins in the following folders
'/Applications/Camunda Modeler.app/Contents/MacOS/resources/plugins/'
'/Users/${username}/Library/Application Support/camunda-modeler/resources/plugins'
That Resources is written in uppercase in your case might be a problem. Putting the plugin in the following direction worked for me
/Users/niklas.kiefer/Library/Application Support/camunda-modeler/resources/plugins
Cf. Plugins docs