Very strange that this is happening...
Do you have multiple versions of After Effects installed and might possibly be running a version different to the folder that you are installing it in?(this is something that has happened a couple times to people with install-issues)
When the file has been fully unzipped, PixSort.plugin should have a different icon that will imply that you've got the right file to put into "Applications/Adobe After Effects (version)/Plug-ins"(PixSort.plugin within the PixSort.plugin.zip should go in here). And then it should show under "Wunk Effects" just like in the PC version.