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.
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.