In index.template.php, look for:
// How about the [search] button?
if ($context['allow_search'])
echo '<li', $current_action == 'search' ? ' id="active"' : '', '><a href="', $scripturl, '?action=search"><span>', $txt[182] , '</span></a></li>';
Under that add:
// SMF Shop Button.
if ($context['allow_memberlist'])
echo '<li', $current_action == 'shop' ? ' id="active"' : '', '><a href="', $scripturl, '?action=shop"><span>Shop</span></a></li>';