Sku is already assigned to Smarty, so this is pretty easy.
Look in
/common_files/modules/Users_online/stats.tpl
Find:
PHP Code:
<tr>
<td nowrap="nowrap" valign="top"><a href="product_modify.php?productid={$p.productid}"><b>{$p.product}</a>:</b></td>
</tr>
Change it to:
PHP Code:
<tr>
<td nowrap="nowrap" valign="top"><a href="product_modify.php?productid={$p.productid}"><b>{$p.productcode} - {$p.product}</a>:</b></td>
</tr>
I just added the "{$p.productcode} - " to append the sku in front of the product name. If you want it displayed in it's own column you will have to adjust the table.
Hope it helps,