Open index.template.php and search for
// Show the [home] button.
echo '<li><a' , $current_action=='home' ? ' class="current"' : '' , ' href="', $scripturl, '"><span>' , $txt[103] , '</span></a></li>';
and after that add this
// Show the [shop] button.
echo '<li><a' , $current_action=='shop' ? ' class="current"' : '' , ' href="', $scripturl, '?action=shop"><span>Shop</span></a></li>';
Now search for this
if ($context['current_action'] == 'search2')
$current_action = 'search';
Then add this under
if ($context['current_action'] == 'shop')
$current_action = 'shop';