How do you give dev rw permission for a group if "mythuser is already in a cdrom group"

As root: for example:
However you may also need to correct the group ownership:

I had this problem too only I edited my fstab to point to dev/sdc0 instead of /dev/hdc or something similar

But for other having this problem running xine-check out of an xterm will run a diagnostic and tell you exactly what you have to do.