I have a canadian rrif and I got an nr4 form. I entered this in turbotax, but I don't see how to enter the tax credit (they take out 15% at source)

No. When you enter the gross amount of the lump sum distribution on Form 1116, this won't double your income. This income is recorded on Form 1116  is information to determine your foreign tax credit for the year since TurboTax does not have the ability to report your foreign pension amount already reported as income on your return to transfer to Form 1116.