Driver?
Nope. He's feeding the mac with an optical signal, that's platform independent and doesn't need any driver.
You should mention Cubase in the subject head, because it's probably software related.
I didn't know the macbook had digi-in, but reading the specs, it says "combined optical digital input/analog line in, combined optical digital output/analog line out"
http://www.apple.com/macbookpro/specs.htmlAre there any settings in Cubase to select source, or switching from analog/digital input?
I know this sounds far fetched, but dud optical cable? Even though the system can recognize the signal, Cubase might ignore it if the cable drops samples. Had a similar issue with my UA-5 > H120 last year.
Last resort, try hooking it up with USB instead. Then you'd need a driver though.