When all else fails look at the form with instructions ... always make the fed payment to "UNITED STATES TREASURY" and mail to the correct address depending on where you live ... https://www.irs.gov/pub/irs-pdf/f4868.pdf
If you have already filed the return then you should use the payment voucher instead : https://www.irs.gov/forms-pubs/about-form-1040-v
Of course you can pay directly online as well : https://www.irs.gov/payments
