The most common kind of digital
audio connection is S/PDIF. The connectors either look like:
this (an RCA cable,
just like the analog one):
this (mini-TOSLINK)--new Macs accept these.
(you don't have to copy this part):
connector pic from
pic from (http://www.cablesdirect.com/prodimages/FCTKMP-A_LR.jpg)