First, check your tax documents to verify that you indicated that you would pay through a direct debit. You may need to login and resave your file with 'all schedules and worksheets'. Then look for a page titled 'Federal Information Worksheet' and look at Part V. This will list your banking information and whether you agreed to a direct debit.
If you find that there is no banking information listed, then the default is that you would pay by check. If this is your case, you can pay the IRS directly on its website here.