scripts/newpost.inc.php

find:
PHP Code
 if ($postername != $ubbt_lang['ANON_TEXT']) {
		$addtofav = "
		<input type=\"checkbox\" id=\"dofav\" name=\"dofav\" value=\"1\" class=\"form-checkbox\" />
		<label for=\"dofav\">{$ubbt_lang['ADDTOFAV']}</label>
		";

change to:
PHP Code
 if ($postername != $ubbt_lang['ANON_TEXT']) {
		$addtofav = "
		<input type=\"checkbox\" id=\"dofav\" name=\"dofav\" value=\"1\" class=\"form-checkbox\" checked=\"checked\" />
		<label for=\"dofav\">{$ubbt_lang['ADDTOFAV']}</label>
		";

if you want to do same for reply, then rinse/repeat with scripts/newreply.inc.php

2c