Assuming that the OP doesn't want to buy anything, another way would be to ask around looking to borrow any kind of digital audio recorder: Tascam, Zoom, and the like. Then cable the stereo's output to the recorder's analog stereo inputs, and let the recorder handle the A-D conversion.
When you're done, pop out the SD card or connect the recorder via USB, and you'll have a WAV or MP3 audio file on your iMac. The advantage is that you don't have to fuss with any software in the transfer chain.
I used this technique for digitizing dozens of cassette tapes, and it worked perfectly. Later, I edited with an audio editor (many choices there, as others have said).
This is hardly the only technique, as is obvious from the other postings.