Mail it to the address that a form 1040 without payment would go to for your state :
https://www.irs.gov/uac/where-to-file-paper-tax-returns-with-or-without-a-payment
And if you have not yet done so, set up an auto withdrawal for the RMD to happen every year sometime during the year prior to mid December so that this will never happen again. This is also what you will tell the IRS on the note you will send telling them how this error will never happen again.
