You'll enter the overpayment from 2015 by clicking on the Federal Taxes tab, then Deductions & Credits, then scroll down to Estimates & Other Taxes Paid and select Other Income Taxes.
You'll then see various category headings for payments made, one of which is 2015 Refund Applied to 2016. The first item in this section is "2015 refund applied to 2016 federal taxes." This is where you'll enter the 2015 overpayment carried forward to 2016.
