Please don't everyone tell me I'm the only one who hasn't figured this out yet . . . but I've always clicked on the last page number listing then scrolled down to see the last (newest) posts.
I just realized that if you click on the date in the "Last post" column, it will take you to that last post . . . no scrolling needed.
Wait, I think we're talking about two different things here.
#1: Setting up the default so newest posts are at the top. That's the way I have mine set up too.
#2: If you've decided not to set up the forum so newest posts are at the top, you can click on the time under the Last Post column on the main page of the forum, and it will jump to the bottom of the last page, the newest post.