In most forums, when there is a "new" tag in the list of discussions, there is a link behind it that takes you to the first unread post. Really missing it here.
If you just click the thread subject that's what you get.
If it says "new" then the whole post is new. If it says "5 new" then there are new comments and clicking on the subject should take you to the new comments. It's not the most intuitive interface.