I think I'm looking at a review (here:
https://head-fi.org/articles/the-many-cables-of-the-ifi-idsd-black.19651/) for example, when I scroll all the way down to the bottom there's a comment box (similar to the "reply to thread" box for threads) that I could type stuff into and then click "Post Comment." Same box appears on the Buying Guide's most recent iteration (
https://head-fi.org/articles/head-fi-buying-guide-travel-headphones.19628/) so I think that's all it is - just scroll down and find the comment box and go for it. If there's some restriction based on your post count or join date or whatever, I have no idea about that (but you don't look like a brand new member so I can't imagine there would be, but really I have no idea).
EDIT: Something else to check - make sure the software didn't sign you out. I honestly can't begin to count how many times the new software has signed me out for no reason whatsoever (other than it's bad at what it does) and that will remove ability to post.
EDIT pt 2: checked a "review" from the front page of the site (
https://www.head-fi.org/showcase/nocturnal-atlantis.22510/) and it seems to use yet another different aspect of this new site software, which doesn't appear to allow comments on the reviews section (because I think those "reviews" are meant to be comments themselves, against the "product page" thing), but if you go over to "General" on the tab (near the top) instead of "Review" and you can see a comments section there.