You'll find the new posts button on the menu near the top of the forum, underneath the header image. This brings up a list of all the posts that have been made since your last visit. Once you have read a post it will disappear from this list. To find it again, just click on the 'recent posts' link on the right hand side of your new posts list. That will show you all the most recent posts, even ones that you have opened and responded to.