Take a line output from the cassette player or the receiver (NOT the speaker output!), and use a 2xRCA to mini stereo plug cable to put it into the line in of your Mac. Use a program like Audacity or GarageBand to capture the sound, then burn it to a CD as audio files. Importing the tracks into iTunes is a convenient way to assemble and burn them.
The next step up is to get an interface, starting with the Griffin iMic for about $60 and going up from there, which can get you better conversion quality.
The iMic has some convenient software called Final Vinyl for importing music.