You should have a 1099R (with code of 4 in box 7) and you will enter it exactly as you have received it in TurboTax.
TurboTax will then ask you questions regarding it to determine the correct tax on it.
It will go under:
Federal Taxes
Wages and Income
Retirement Plan and Social Security
IRA, 401K, Pension Plans (1099R) you will click add or update.
