Standard CD Rom Drives all have this little hole on the front. You have to use a bended paper clip, push it carefully into the hole, the drive should then open even when the machine is off. That said, I wouldn't be surprised if a mac has something else than a standard CD Rom drive (maybe something pricey very special)
Except the Mac 'form over function' mantra meant they didn't bother to include the ejection hole on many/most of their lineup.