I'm here with the same question. I talked to someone (Terri) via live chat about this while doing my taxes and she said that instructions would be emailed. So I e-filed my returns.
My state return was accepted, and I've yet to receive any instructions on what to do about this proof.
On further research I came across this post that says we can't e-file for this situation:
https://ttlc.intuit.com/community/after-you-file/discussion/i-qualify-for-the-maine-opportunity-cred...
In your case, I'd probably file your state by regular mail and include the proof with the return. I wish I'd done that. I have no idea what to do now.
