My FD1040 has a business in the home (8829) with Sch C. Additional mortgage interest should carry to Sch A, but I cannot find where to enter the data to make it work. ??

To enter personal mortgage interest, search for 1098 and use the Jump to 1098 link.   The deduction goes to Schedule A, but that schedule gets sent to the IRS with your tax return ONLY if you itemize deductions.    It is in the Your Home section of Deductions and Credits.