I'd suggest undo-ing what you did following that post. The template you posted is the Whos Online page.
Can you post your forum URL?
If this is the template contents for the plugin(posted in other thread):
<td class="tcat"><strong>{$lang->whos_online_today}</strong> [<a href="online.php?action=today">{$lang->complete_list}</a>]</td>
</tr>
<tr>
<td class="trow1"><span class="smalltext">{$lang->online_note_today}<br />{$onlinemembers}</span></td>
</tr>
Replace it with this:
<td class="tcat" colspan="4"><strong>{$lang->whos_online_today}</strong> [<a href="online.php?action=today">{$lang->complete_list}</a>]</td>
</tr>
<tr>
<td class="trow1" colspan="4"><span class="smalltext">{$lang->online_note_today}<br />{$onlinemembers}</span></td>
</tr>
Notice the "
colspan="4"" on
both <td> elements.
As a bonus, here's a cool little "trick" to help you figure out what templates are used where in the future. I'm using Google Chrome here, but I'm sure most browsers support inspect element?