You've been able to dig up some very old screenshots, well done :) It looks a bit different now, but it's still found in the same place. It is part of the 'flag dialog': under each post, there's a list of links, and 'Flag' is one of them:
Choose 'a duplicate' (of course, this option is only available for questions, not for answers):
But, to be honest, this duplicate finder is not really good at finding duplicates. I always use the regular search and Google or other search engines are even better (or so I heard). I see now you received some suggestions already in the comments to your earlier question.
Do I need to be a certain reputation to access it?
Yes, flagging requires 15 reputation, so you'd be able access it here, and on Photography Stack Exchange. You don't need to have the privilege to cast close votes, but if you do, you can also reach the duplicate finder via the 'Close' link.