I couldn't find a reason why the software doesn't see the SD card. I believe it hasn't been properly formatted. The best way to overcome this issue is with the help of the Disk Utility found in all Mac OSX installations. Start Spotlight and type Disk Utility. Locate the SD card on the left side and use the Erase function to wipe it clean, then restart dd Utility and check if it's present.
One reason for this error might be related to support and compatibility. The dd Utility has support only for limited SD cards and storage devices.