The previous post about sorting was regarding the order of the posts INSIDE A TOPIC. For those users who are still having trouble getting the posts to show up in the list sorted from newest to oldest when the are looking at a LIST OF TOPICS, try this:
In each board you are given a list of topics/discussions. In the header of the list there is "Last Post" with a little arrow beside it. The direction this arrow is pointed will determine if the posts are sorted. Pointing down, as shown in the image below will sort the topics from most recent to oldest (EXCEPT PINNED topics, which always appear at the top). Clicking the arrow will reverse the direction, with oldest topics on top.
BE AWARE you may need to click the arrows a few times to get the sorting to appear as you wish. This is a known bug.