You need a toslink to mini plug cable. The mini plug end goes straight into the MBP's hp out. I found toslink to sound better (clarity and coherence) than usb on all the macs I've owned when employing the Bifrost/Gungnir for dac duties. Some may prefer the warmth and more intimate presentation of usb though.
This is not to state optical is superior to usb from an MBP. It's more reflective of Schiit's implementation. I greatly prefer usb feeding my NAD M51 over toslink from my MBP.
Cool, thanks. I guess what I'm getting at is whether it is worth the extra $100 for USB, heh.