I filed my Michigan return electronically and on the print out of the return it says that I have to sign and date form MI-8453 within 3 days of acceptance. What is this?

If you filed them both electronically, this does not apply to you. If you mailed your Michigan return, there would be an actual signature on the return.

Form MI-8453 is the declaration and signature document for a State Stand-Alone (unlinked) return. If you e-file your federal and Michigan returns, Michigan will accept the federal signature (PIN). If you e-file a State Stand-Alone return the Electronic Signature Alternative (ESA) or paper Form MI-8453 must be used to sign your return.

Where do I find a form MI-8453?