It sounds like you're trying things from a windows perspective. In linux in general you don't change user privileges. The user has complete control of everything in their home directory, but can only
execute programs, not change them. the proper way to do things is to change the file permissions. Check out his link:
Now as to your problem. If you haven't screwed up things to badly, you should in a terminal be able to enter the following commad and eject your cd.
sudo umount /media/cdrom [enter]