Well first of all you do not mount a cd to burn it. Secondly you likely have to configure K3B before it will work. Sorry I have no use for a GUI to burn cd's so I'm of little help there. Search around here to see how they do it.

Is your cdrom correctly identified?

ls -al /dev/cdrom

Most Linux apps will expect your cdrom to be named /dev/cdrom. If not just link it.

ln -s /dev/scd0 /dev/cdrom