Enter it in the Federal Taxes tab,
Deductions and Credits,
Choose What I Work on if you see it,
if not, just scroll down to mortgage interest.
Enter the name of the lender, check anything that applies on the next screen.
Enter the deductible amount you calculated and check the box below that says "interest amount I entered here is different from what's on my 1098".
The next screen lets you enter an explanation.
