The plugin is set to order these products by original products' sort order which you set in admin (although kinda useless if you have products from various categories) and then by products' names in alphabetical order.

If you want to change that and have the products show in the exact order as you entered the IDs in your admin, you'll need to make a change to one file - includes/templates/YOUR_TEMPLATE/templates/tpl_index_slider.php.
Find
Code:
case 'custom':
            $products_query = "SELECT DISTINCT p.products_id, pd.products_name, p.products_image, p.master_categories_id FROM "  . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd WHERE p.products_id IN (".ZX_CAROUSEL_MOD_CUSTOM.") AND p.products_status = '1' AND p.products_id = pd.products_id AND pd.language_id = " . (int)$_SESSION['languages_id'] ." ORDER BY p.products_sort_order, pd.products_name";
            if ($products_query != '') $products = $db->Execute($products_query, ZX_CAROUSEL_LIMIT);
            break;
and replace with
Code:
case 'custom':
            $products_query = "SELECT DISTINCT p.products_id, pd.products_name, p.products_image, p.master_categories_id FROM "  . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd WHERE p.products_id IN (".ZX_CAROUSEL_MOD_CUSTOM.") AND p.products_status = '1' AND p.products_id = pd.products_id AND pd.language_id = " . (int)$_SESSION['languages_id'];
            if ($products_query != '') $products = $db->Execute($products_query, ZX_CAROUSEL_LIMIT);
            break;