If you're trying to update Ubuntu, the CD won't help you unless you are updating from a previous version - however you'd be better off updating through the network.
Anyhow, if you are just trying to do normal system updates, follow these steps:
sudo nano -w /etc/apt/sources.list
Comment out (using the # symbol) any line that has CD ROM in it (usually at the top of the list)
After that, update your sources and update your software:
sudo apt-get update; sudo apt-get upgrade