you need make a "link" in your forum, in the template with you use....
generally is in the index.template.php on the line 687 (see the buttons)
i have in my forum...
//Shop
echo'
<td align="center" class="menubg' , $current_action == 'shop' ? '2' : '' , '">
<a href="', $scripturl, '?action=shop">RulzStore</a>
</td>';
sorry my english XD