If this is my first time filing Credit for Educational Opportunity for Maine, where do I send proof of loan and payments?

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-credit-how-do-i-attach-a-copy-of-my-transcript-and-proof-of-my/00/725172#

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.

Yes, if you must attach separate documents, you will need to print and mail in your state tax return along with the requested "proof".  If you have already e-filed your ME return contact the ME Department of Revenue and explain.  They may have you complete an amendment to apply for the credit after your original return is filed.