The headphone jack is a combined analog and digital audio out. For something simple (going to a TV or some small powered speakers) you can just use a standard 3.5mm headphone connection and maybe a 3.5mm to RCA adapter depending on what you're connecting to.
For digital out or surround sound there's also an optical audio out and you'll need a Toslink cable that has a regular 5mm connection on one end (for your audio equipment) and mini Toslink connection on the other end (for your Macbook). You can either get a cable like that or just get a regular Toslink cable with a 5mm to mini adapter to connect to your computer. Here's a cable from Monoprice:
http://www.monoprice.com/products/pr...seq=1&format=2
Obviously you'd also need something that can handle an optical audio in for the 2nd option to work.