Deductions & credits

• Open or continue your return in TurboTax.
• In the left menu, select Tax Tools and then Tools.
• In the pop-up window Tool Center, choose mortgage interest worksheet
• If you can't enter the principal amount borrowed,  click left on the entry box
• a pop up will appear and ask for the principal amount
• after entry just close the pop up and the entry will appear now on the worksheet

Not very user friendly