While entering your form 1099-R, if you have entered a birth date which makes you older than 70 1/2 in the tax year, TurboTax will ask you if any portion of your distribution was transferred to charity. You then enter the amount transferred to charity.
The code in box 7 of form 1099-R must be 7 and the box IRA/SEP/SIMPLE must be checked.
TurboTax will then reduce the taxable amount by the amount transferred to charity.