You probably don't need any special driver for the serial converter, most will work out of the box. Has the USB serial device node been created? There should be a file along the lines of /dev/ttyUSB0 created after you plug in the device, and you can also check the output of the kernel with a command line "dmesg | tail".
Assuming that the adapter is working, you need to configure minicom to use /dev/ttyUSB0 (again, assuming this is correct for your system), and set the rest of the parameters however the Cisco device expects.
What is the exact problem you are having now? Is there an error? Garbled text?