Brad- click on "User CP" in the blue bar near the top left of your page. Scroll down to "Settings and Options".

Click on "Edit options".

When that opens just scroll down to the "Thread Display" box.

Look for the
"Thread Display Mode" area and click on the drop down menu.

Choose the "oldest first" menu choice.
Finally scroll down to the bottom of that page and click on "save changes" button.

You're good to go.