What do I need?

Your macbook should have a dual audio/toslink output. You need a mini jack toslink to toslink cable. Silflex glass works great for about $70. Confirm, but all Macs have this output.

Alternatively, you will have to buy a USB to RCA/P/Dif converter. They can be had for a few hundred bucks.

Meiwan is right, any Macbook Pro has a mini-Toslink/optical output inside the dual-purpose headphone jack. If you can't easily find a mini-Toslink to Toslink cable you can use a mini to Toslink adapter on a standard Toslink cable. The adapter is simply a spacer and doesn't have any effect on the optical signal. You might be able to find an adapter at Radio Shack but, if not, Monoprice is a good source.

Yep, the other guys are correct. The headphone output has a dual purpose. It is also a mini-Toslink optical connector. Get a mini-Toslink to Toslink optical cable or a mini-Toslink to Toslink adapter plus and a sepaerate Toslink cable.

You might want to verify the above, but I'm 90% certain the headphone output is a dual purpose type.