The jack you want is the one with the two triangles, which is the 'line in' port on your iMac. After you plug the cassette player into it you will need to go to the System Preferences (located under the Apple menu in the top left corner of the screen). From there go to the "Sound" panel (2nd row at the end). Click the "Input" tab and select "Line In" instead of the "Internal microphone". That's it. You should be able to record from the cassette player now.