Another approach would be to purchase a separate interface unit that provides an analog-to-digital conversion function, and outputs the digital data via one of the standard computer-compatible digital interfaces (USB, Firewire, etc.). A vast number of such devices are available, at all kinds of price points:http://www.bhphotovideo.com/c/search?Ns=p_PRICE_2|0&ci=14834&N=4294550051&srtclk=sort
I have no Mac experience, but in the PC world I would expect that one of these devices, if well chosen (be sure to review the user comments on any device being considered) would provide much better sound quality than a typical inexpensive consumer-oriented built-in sound card (or equivalent circuit on the computer's motherboard), in part because of inherent differences in their quality, and in part because of the proximity of the built-in card to digital noise in the computer. (I realize we are talking about cassettes, with their inherent quality limitations).
Rodman -- when you referred to the computer's "stereo mic input," didn't you mean the computer's "stereo line input," the mic input probably being too sensitive?
Learsfool -- On desktop PC's, at least, the stereo line input jack is color coded light blue, and will be located near the lime green line-out jack that is probably connected to the speakers. This
is an example of the required adapter, if the built-in sound card approach is chosen.