Fortunately, your Federal tax refund from last year is not taxable. You do not need to report this on your current year tax return.
If, however, you applied your refund from last year to your taxes for this year, you should report this under Deductions & Credits >> Estimates >> Other Income Taxes (see screenshot below - click to enlarge).
The easiest way to find this section of TurboTax is to use the Search box at the top right side of the TurboTax header. Click on the magnifying glass, type in "estimated taxes paid", and hit Enter. The first result should be "jump to estimated taxes paid". Click on this link to go directly to the input screen for this topic.