Ubuntu 12.04 comes with a bug that it will not mount the cdrom on proper path and that prevents installation of applications from the Ubuntu Repositories even if you add the repositories via apt-cdrom successfully.
Whenever you try to install applications you'll get an error message like
E: Unable to stat the mount point /media/cdrom/ - stat (2: No such file or directory)
Here is how to fix this problem:
Open the Terminal and follow the instructions below to add the Ubuntu repository DVDs to the Software Sources list (sources.list).
Create a mount point for cdrom device (if the directory already exists, goto the next step)
sudo mkdir /media/cdrom
Edit the fstab to make some change.
sudo gedit /etc/fstab
Paste the following line at the end of the file
/dev/sr0 /media/cdrom udf, iso9660 user, noauto 0 0
Save and close the file.
Then restart the system.
After restarting the system, insert the repository DVDs one by one.
Then copy and paste the below command to add the DVD to the source list.
sudo apt-cdrom add
You'll be prompted to name the DVD. Give the name such as DVD1, DVD2, DVD3...etc.
After all the DVD completes, reload the system repository using following command.
sudo apt-get update
Now your system is ready to install any package using the Ubuntu Repository DVDs.