you can use
template conditionals plugin AND modify below code in
index template
<a class="buttons" style="float: right;"></a>
<a class="clickedbuttons" style="float: right;"></a>
<br class="clear" />
<div class="sidebar" style="float: right;width: 19%;">
<table border="0" cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="tborder">
<tr>
<td class="thead"><strong>
Latest News
</strong></td>
</tr>
<tr>
<td class="trow1">
You can put some news stuff here
</td>
</tr>
</table>
</div>
<div class="forums" style="float: left;width: 80%;">
{$forums}
</div>
add
<if $mybb->user['uid'] > 0 then> at the top of above code AND add
</if>{$forums} at the end