You hade a good start, but did not finish the job.
You can see, in your file, that you have connected your forum button to help
// Show the button.
if (empty($context['disable_sp']) && in_array($modSettings['sp_portal_mode'], array(1, 3)))
echo '<li><a' , $current_action=='forum' ? ' class="current"' : '' , ' href="', $scripturl, '?action=help"><span>' , $txt['sp-forum'] , '</span></a></li>';
Instead, I recommend you copy the code from the tutorial, and use
// Show the [forum] button.
if (empty($context['disable_sp']) && in_array($modSettings['sp_portal_mode'], array(1, 3)))
echo ($current_action=='forum' || $context['browser']['is_ie4']) ? '<td class="maintab_active_' . $first . '"> </td>' : '' , '
<td valign="top" class="maintab_' , $current_action == 'forum' ? 'active_back' : 'back' , '">
<a href="', $scripturl . ($modSettings['sp_portal_mode'] == 1 ? '?action=forum' : ''), '">', empty($txt['sp-forum']) ? 'Forum' : $txt['sp-forum'], '</a>
</td>' , $current_action=='forum' ? '<td class="maintab_active_' . $last . '"> </td>' : '';