1) I successfully imported multiple files on the backend, however, newer products are not showing on the admin panel as well as the front end.
2) I am unable to integrate the payment gateway as well. I have already uploaded the required files via FTP, but it is not reflecting in System -> Configuration -> Payment Methods.
1. Without tax_class_id field and status fields products might not be visible in back- and front-end. Check the file you imported and make sure that the mentioned fields were included and they were formatted according to Magento requirements. Here you can check format -