The tag below your question says that you are using TurboTax Free Edition. But you cannot enter a Form 1098 in Free Edition. Mortgage interest is an itemized deduction. It requires Schedule A, which is not included in Free Edition. You have to upgrade to Deluxe in order to enter Form 1098 or any other itemized deductions.
At the very bottom of the screen where you enter your 1098, click the link that says "What if I have something in Box 4 of my 1098?" It will give you instructions for what to do. You might have to scroll down to see the link.
