If you have an internet connection, you can safely comment out the line starting with deb cdrom: .... in the file /etc/apt/sources.list. You can edit this file with the command:
sudo gedit /etc/apt/sources.list
and simply prepend this line with the #-sign so that it looks something like this:
# deb cdrom:[Ubuntu 12.10 _Quantal Quetzal_ - Release amd64 (20121017.5)]/ quantal main restricted
Save the file and don't forget to run
sudo apt-get update
from: http://askubuntu.com/questions/236288/apt-get-asks-me-to-insert-cd-why