The additional state tax you paid with your 2015 return is deductible in 2016 as an itemized deduction on Schedule A.
You'll enter it 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 be taken to the Other Income Taxes page where you'll see various headings. Under Other Income Taxes Paid in 2016, you'll make your entry under "Balance due on your 2015 state taxes paid in 2016."
