That's very strange that it doesn't show up there. Even OSX is regarding it as a valid plugin but After Effects refuses to recognize it and you mentioned it happening to a previous version of After Effects as well...
Just one more thing to make sure:
on that .plugin file you should be able to right-click>Show package contents to browse the contents of the plugin archive. Within it, verify that the contents look like this:
Particularly that the "PixSort" file is within the MacOS folder as it might imply that you have an older download of the plugin or the initial extraction did not work correctly.