You need to save/print your completed and accepted return in PDF format. (If that's allowed on MACs; I don't know)
IN the printout will be an "electronic receipt" of acceptance by the IRS. The receipt will show the date/time you filed it, along with the date/time the IRS accepted the e-filed return. That is your primary proof that you filed it, and that the IRS accepted it as filed.
Do take note however, that just because the IRS has accepted the e-filed return, does not mean they agree with it. So if it's been 90 days since you filed, there may be an issue beyond your computer that is delaying any refund you may be due. For example, if your W-2 employer has not submitted their form 940 showing wages paid and tax withheld, that can delay things and there's no much that "you" can do about it but wait.
However, if for any reason in the future the IRS states you did not file or that you filed late, then that "electronic postmark" printout is your proof that you did.
