What do I need?

I've got a Mcbook pro that has US outputs. I'd like to get signal from the Macbook to my Cambridge 840c which has P/Dif and Toslink inputs. What little gadget do I need to make this happen?

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.
Forget the toslink output from the Mac. High jitter, low sound quality.

Get a good self-powered USB to S/PDIF coax converter that works asynchronously up to 24/192. Make sure it has a good clock. This is actually more important than the DAC itself. Then get a good 1.5m S/PDIF cable to connect it to your DAC. The Ridge Street Audio is such a cable. I dont sell them and have no affiliation.

The clock in the Mac is not good enough for generate a low-jitter S/PDIF signal.

It's like a cartridge on a turntable. The master clock is where you want to put your money in digital audio.

Steve N.
Empirical Audio
Just to follow up, I went with a Musical Fidelity V-Link using a toslink connection to my CDP. Stupidly easy to use and it actually sounds pretty good with some music I'd uploaded from the original CDs into Apple lossless. The Lossless files are a slight sound degradation from the original CDs, but very slight, and for a mix for a party, more than acceptable.

It sure beats making mix tapes on cassette like we did in the old days.