what kind of mac do you have? if you have a mac with audio in, then you can most likely download a shareware/freeware program for os x and then run the audio (using a standard 1/8" to 1/8" cord) from stereo to mac. the program would then capture the audio.
if you're just looking to score some music, streamripper lets you save streaming mp3 audio from online radio stations. itunes has a big list of stations.
if your mac doesn't have audio in, you'll need something like the iMic, which has various audio inputs.