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Please find the form 1116 here - http://www.irs.gov/pub/irs-pdf/f1116.pdf and instructions here - http://www.irs.gov/pub/irs-pdf/i1116.pdf
If the same income is taxable in the US and in a foreign country - you might be able to claim a credit for foreign taxes paid on your US tax return. The form 1116 is used to calculate the amount of the credit and should be attached to your US tax return.
The credit will be claimed on the form 1040 line 47 - http://www.irs.gov/pub/irs-pdf/f1040.pdf
If you did not claim a credit in previous years - you may amend your part tax return and claim an additional refund. Generally - you have three years to claim a refund before the statute of limitation runs out.
Please be aware that if the distribution from the trust is more than $100,000 - additional reporting is required.
Let me know if you need any help or clarification.