Open index.template.php and search for
// search button
$context['menubox'][]=array(
'title' => $txt[182],
'link' => $scripturl.'?action=search',
'chosen' => 'search',
'memberonly' => false,
'guestonly' => false,
'permission' => '',
);
Add this code after that one
// shop button
$context['menubox'][]=array(
'title' => Shop,
'link' => $scripturl.'?action=shop',
'chosen' => 'shop',
'memberonly' => false,
'guestonly' => false,
'permission' => '',
);