Make sure you reload the page so that you load the latest code. You’ll see it automatically on any stories that get clustered, since it’s automatically turned on for all users. If you’re not a Premium Archive subscriber, then you won’t see it on as many of your feeds, because you’re only benefiting from other Premium Archive users who are subscribed to the same feeds as you are.
That said, you can see the options under the feed options pop over, which is in the top right of the screen when you open a feed, and you can configure it there. You can also enable and disable it in the global preferences dialog.