You need a USB to RS-232 adapter cable, Celestron model 18775, or its equivalent. As of mid-2012, the 18775 is unavailable.
Here are two adapters recommended by Celestron in the aftermarket.
TrendNet TU-S9. Available from Amazon.com. It uses a Prolific chipset.
KeySpan USA-19HS. Available from TrippLite or Amazon. It uses a Texas Instrument (TI) chipset.
Both adapters work with operating systems Windows 7 (64-bit), Windows XP (32-bit) and Mac OS X 10.7 Lion. Both adapters work with MCupdate, HCupdate, and the new Celestron Firmware Manager (CFM) on all 3 operating systems. Most needed drivers are right on the supplied disks with the adapters; you may need to go to the manufacturers websites for the most current drivers (for example, Mac OS X Lion).
The Keyspan is a more expensive adapter, however it has the following useful features:
Detachable USB cable. (This allows you to buy a separate cable that is as long or short as you need.)
Status lamp. It glows steadily when there is power, blinks when the drivers connect (ready to use) and fast blinking when there is traffic on the serial bus.
KeySpan serial assistant software that links up to the driver. It shows detailed diagnostic data and provides self tests, shows all RX and TX traffic (HEX or ASCII modes) on the serial bus at any baud rate, shows setting changes such as baud rate and even DTR/DSR/CTS/RTS. All of the above information can be logged saved for later use as well.Assigned COM port number doesn't change when the adapter is plugged into a different USB port.