Option 1 – http://www.griffintechnology.com/products/imic/ – The iMic is a low cost device $30 that allows stereo audio in. It comes with software or you can use other software to make edits like Garageband for example.
Option 2 – http://www.costco.com/Browse/Product.aspx?Prodid=11239293 – The Ion Audio Dual for $120. This piece of hardware is Mac or PC compatible and even has settings for CrO2.
Option 3 – You can plug in the audio out of the cassette player to the line in on the computer. This might result in noise, faint sounds, or reduced quality (due to the DAC conversion) depending on the quality of the cable you use and/or the specific characteristics of each of the units. I would not recommend this.
Option 4 – You can plug in the headphone jack to the audio in and this will also result in noise and reduced quality to another DAC conversion. I would not recommend this.