Well, than that's the problem. If it isn't in AMS, that means that Mac OS doesn't see it. That could be a (silly simple or supercomplicated) hardware issue.
The first thing to check is the seating of your USB plugs, second exclude that it is the USB port on your computer by hooking up another piece (memorystick, MIDI controller etc) known to work on that USB port, or third by hooking the Presonus to a different USB port. If the USB ports all work but the Presonus works on none of them, switch out the cable (for one you know works). If still nothing, fourth: check the Presonus' software, if it is up to date and if things are set correctly. Also doublecheck any switches on the box itself.