Enter the pension at the very bottom of the wages and income section as 'Miscellaneous Income', 'Other Reportable Income'. You can write your own description there so just enter the description as 'Japanese Pension' and then the amount.
If you were required to pay taxes on the pension in Japan then you can deduct the taxes paid as a foreign tax credit in the deductions and credits section. If not then the entire amount is taxable to you on your US return.
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