Open index.template.php and search for
if ($context['current_action'] == 'search2')
$current_action = 'search';
Add this under that code
if ($context['current_action'] == 'shop')
$current_action = 'shop';
Now search for this
// How about the [help] button?
echo '<li' , $current_action=='help' ? ' class="chosen"' : '' , '><a href="', $scripturl, '?action=help" ><span>', $txt[119], '</span></a></li>';
Then under that add this
// How about the [shop] button?
echo '<li' , $current_action=='shop' ? ' class="chosen"' : '' , '><a href="', $scripturl, '?action=shop" ><span>Shop</span></a></li>';