Hi Rod
Threads should be reading oldest to newest by default. If they're not, it's simply a matter of going into the control panel (accessible from the front page of the forum), then "edit options" ( a shortcut is:
http://www.dotable.com/profile.php?do=editoptions )
About half way down the page is the section "Thread Display Options" where you can change the option "Thread Display Mode" to "Linear - Oldest First".