If it's a cdrom that you're trying to add to your repositories in the /etc/apt/sources.list file, then the command is "apt-cdrom add". After entering this command, you'll be prompted to add the cdrom, and then it will be added (these commands likely will have to be run with sudo).
What is the mount point for your cdrom (/dev/hdc) in your fstab file (/etc/fstab)? Mine is:
# <file system> <mount point> <type> <options> <dump> <pass>
/dev/hdc /media/cdrom0 udf,iso9660 user,noauto 0 0