Ubuntu should auto mount when you insert a disc. You can't mount the DVD/RW without a disc because there is nothing to mount. If it isn't auto mounting then try :-
Code:
sudo mount /dev/hd[letter_of_device]
mount will try to guess the file type. Otherwise
Code:
sudo mount -t [file_type] /dev/hd[letter] [mount_location])]
See man mount for file types.