The audio output port on a Macbook is a combination analog and digital output. You can plug a stereo-mini analog cable into it to get output for headphones or to go to the analog input of some other amplifier.
To use it as a digital output you would need an optical mini-Toslink to Toslink cable, either glass or plastic fiber, which will transmit the digital stream using the Spdif protocol to a digital/analog converter.
If you go that route, which I do, you can improve the sound quality quite a bit by using a reclocker between the Macbook's digital output and your DAC. Wyred4Sound makes a very good one, the Remedy Reclocker, and there are others. The reclocker isn't required but will help with the significant amount of jitter in the Macbook's digital output.
To use it as a digital output you would need an optical mini-Toslink to Toslink cable, either glass or plastic fiber, which will transmit the digital stream using the Spdif protocol to a digital/analog converter.
If you go that route, which I do, you can improve the sound quality quite a bit by using a reclocker between the Macbook's digital output and your DAC. Wyred4Sound makes a very good one, the Remedy Reclocker, and there are others. The reclocker isn't required but will help with the significant amount of jitter in the Macbook's digital output.