If your e-file did not go through and you are printing and mailing in your return, you should include your payment with your mailing (in addition, to any W-2s and any other documents for your return). If the e-file did not go through, the tax payment did not go through. If you are mailing in a check to the IRS, it should be made payable to "United States Treasury".
The IRS has some tips on how to prepare your return for mailing. Click on this link for more information: IRS How
to Prepare Your Tax Return for Mailing
Be sure to keep a copy of your return for your records.