Verify that the USB to Serial Adapter connected to the K3 is the one you have chosen on the Port page.
Mac OS X will list devices other than serial ports (ie Modems and Bluetooth devices). Do not select these devices.
Linux will usually not list USB to Serial devices in the drop-down list. If using a USB to Serial adapter on Linux, type the device name in the text field (ie /dev/ttyUSB0) below the drop-down menu and click the check box next to the text field to connect to the device.