HELP with MAC MINI and PS DIgital III DAC

which input are you using...? Don't think the USB port can support hi rez.

HD Tracks uses flac files which i-tunes cannot handle. Not sure if Amarra works with flac - assume it does. I'm using PureMusic with itunes and I had to do a little fiddling with the settings in itunes to accept flac files. The amarra manual will tell you how. There is mac friendly software for flac files you can get for free, also.

How are the two being connected?

First thing to check for -- if you are connecting the Mac Mini to the PS Audio by USB cable, has the Mac Mini recognized the additional output and have you selected it as your audio output on the Mac? Open System Preferences, select "Sound", and click on the "Output" button to view the selected and available outputs. Your DAC should be one of the available outputs. If it is not already selected, do so, and see if that works. If you don't see the DAC listed as an output, check to make sure your connections are all solidly seated (it's easy to fail to fully insert a USB plug).

Meiwan raises a good point/possibility...Itunes cannot handle FLAC...I use Max to convert from FLAC to AppleLossless, and it's free and easy to use. Once you convert to Alossless, ITUNES will see and process it just like other music in your library....

AMARRA, while great stuff, is only a front end...and works with ITUNES

HOWEVER, you really need to tell us what you mean by "can't play"

Without knowing where you are stuck...it's hard to for us to tell and help you.