Where do I enter monthly pension plan payments?

Pension plan payments you received during the year are reported by the pension plan administrator to you on a Form 1099-R.

To enter, edit or delete a form 1099-R -

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Scroll down to Retirement Plans and Social Security
On IRA, 401(k), Pension Plan Withdrawals (1099-R), click the start or update button

Or enter 1099-r in the Search box located in the upper right of the program screen. Click on Jump to 1099-R

Form 1099-R shows the total amount that you received during the year. It does not show the monthly amounts.

If you don't have the 1099-R, contact the plan administrator and ask for it. You need the 1099-R in order to enter your pension income. You cannot just enter the amount that you received.