If you used the CD/download to prepare your tax return, you need to open the original return, then go to Other Tax Situations. Scroll down on that screen to find Amend a Tax Return.
When you amend you will need to enter the 1099R for the retirement distribution.
To enter your retirement income, Go to Federal> Wages and Income>Retirement Plans and Social Security>IRA 401 k) Pension Plan Withdrawals to enter your 1099R.