What groups do your user belong to ? Please check by typing "id" in a user console.
What exactly have you changed in the K3B permissions and how did you do that exactly ?
Also, please do try as well in a user console:
$ wodim /path/to/your-iso
Where "/path/to/your-iso" is the full path to the iso file (or from within the console, cd to the same directory as the disk image)
(with a CD or DVD ready to be burned, in the drawer of the DVD burner).
One last question : have you ever cleaned the lens of this device ?