You can also enter your instalment payments in EasyStep mode. 

If you are using TurboTax Desktop Basic, click on the EasyStep icon on the blue menu bar, then select Deductions & Credits > Federal Installments from the left side menu. 

In other versions of TurboTax Desktop, click on the EasyStep icon on the blue menu bar, then the Find icon. Search for instalments, select it from the list & click Go.

You can return to the forms by clicking on the Forms icon on the blue menu bar.