Find in your SHOWTHREAD template:
Code:
$vbphrase[closed_thread]" border="0" /></if></a>
Add directly after:
Code:
<a href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" /></a>
If you want the New Thread button to appear next to both New Reply buttons, you'll need to do the above twice -- there is another further down the template.