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.
The entry page on the web has apparently changed and this reply is no longer valid.
If your retirement income is reported on Form 1099-R, then the taxes withheld should be shown on that form in box 4. Entering the Form 1099-R will take care of entering the taxes withheld at the same.
Go to Wages and Income > Retirement Plans and Social Security > IRA, 401(k), Pension Plan Withdrawals (Form 1099-R) to get started.
While you are working in your return, you can also enter the search term '1099-R' in the box that opens when you click the magnifying glass at the top right corner of the screen. This should give you a blue link that says 'Jump to 1099-R'. Click the link to go directly to that section to enter your information.