It's on the bottom of the case below the hard drive bays. Some of the 300 cases don't have it, I think Antec added it later because SSDs became so popular.
I did end up putting a regular 2.5" drive in there (hybrid SSD) and it works fine. You've got to screw it in REALLY tight though because when the HD starts to spin, it will make a rattle noise if there is ANY type of give.