The new 17" PowerBooks have a built-in digital minijack. All you need to do is go to Radio Shack or some similar electronics/audio store and pick up a gold minijack to RCA adapter. They cost about $10. Then just run a short minijack-to-minijack (male/male) cable from the PowerBook and connect your shiny new adapter. From the adapter, use a standard RCA cable or your digital coax cable into the coax port on the Fantom. The RCA cable (like a yellow one included with digital cameras) will carry the digital signal just fine.
I have a similar setup on my PC. My sound card has digital output through either minijack or TOSLink optical, but because my surround sound receiver's optical connection is full, I had to come up with a use for the minijack. It works perfectly (Windows required some coaxing to pass DVD AC3 untouched, but I think it would be less troublesome on the Mac).
I wish the 15" PowerBooks had digital-out! It's one sorely missed feature of my new one.
EDIT: Oops! I just realized that you want to input to your PowerBook, not output to the keyboard. I don't know what I was thinking! You could use the Phantom's USB connection or the standard headphone port, since the PowerBook does NOT have digital *input*.