If downloading your images fails no matter which protocol you have chosen (PTP or USB), try using the command line instead of Digikam. The gphoto2 --P command initializes the camera and downloads the images, if gphoto2 supports this camera.

Try gphoto2 --list-cameras and the information at http://www.gphoto.org/ to obtain more information on the status of your camera's support by Linux.

If the camera is not listed there, remove the storage medium from the camera and use a card reader device (either an external or internal one). As soon as the medium is detected it is mounted automatically and you can import the images to your Digikam collection with Albums+Import+Add Images or Albums+Import+Import Folders.