Foreign taxable income not included in W-2

I got a distribution of my pension funds in a foreign country (which is treated as Other Income here) last year and this year which were not taxed in the foreign country and not included in my W-2.  For the income tax filing for last year, I was penalized because I had not paid the extra income tax in U.S. due to the distribution.

Shall I pay the estimated extra income tax before the end of this year?  If yes, how?