That is entered when you enter all of the boxes from your W-2.  You do not enter it anywhere else unless you took money OUT of it.    ( It will be in box 12 a-d)   Just enter you rW-2 exactly as it appears.

Or do you get a pension?  You get a 1099R for pension payments.  

If you meant you are getting money from your 401k then in January /early February you will get a 1099R.

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.