Valab Dac Moacbook and itunes

Hi
Ive just bought a Valab DAC which I am trying to use with a Macbook via a USB cable from the Mac into the USB input on the Valab. It works great with recordings Ive made in Garageband; the Valab is immediately recognised by the computer and works well, however I am trying now to play music from my itunes library on the computer but in this application the Valab isnt recognised and the music just comes from the tiny speakers in the Mac book.

Is there a way to get the Mac to recognise the Valab via the USB so I can play my itune library through the Valab?
Any advice greatly appreciated.
Thanks
Ian