I seen how the buttons worked on black metal, now, test this out on your forum.
I simply copied the button info for arcade, but made the changes to link to the shop.
Original Shop button code
//Begin SMFShop code
//loadLanguage("shop");
echo ($current_action == 'shop' || $context['browser']['is_ie4']) ? '<td class="maintab_active_' . $first . '"> </td>' : '' , '
<td valign="top" class="maintab_' , $current_action == 'shop' ? 'active_back' : 'back' , '">
<a href="', $scripturl, '?action=shop">Shop</a>
</td>' , $current_action == 'shop' ? '<td class="maintab_active_' . $last . '"> </td>' : '';
//END SMFShop code
Edit to match other buttons for your theme
// Shop Button
echo '<li><a' , $current_action=='shop' ? ' class="current"' : '' , ' href="', $scripturl, '?action=shop"><span>Shop</span></a></li>';