In your navbar template find:
Code:
<form action="search.php?do=process" method="post">
Below that, you should see...
Code:
<input type="hidden" name="do" value="process" />
<input type="hidden" name="quicksearch" value="1" />
<input type="hidden" name="childforums" value="1" />
<input type="hidden" name="exactname" value="1" />
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
The code segment in red is what you're probably missing, or has been typoed in some way...