The USB cable is for editing, the midi cable sync's to other devices and allows the Minibrute to be used as a controller.
If your iMac as Audio In, or a "Mic" in port, likely a 1/8" female jack, then you may be able to get an adapted cable to do so, then go into the settings folder and make the necessary adjustments.
Other than that, an audio interface is your best bet for getting audio in/out of your iMac, then get a reasonable set of studio monitors. Even some of the cheaper M-Audio ones translate well.
