I method described by dv3500ea does not working now. (I think it worked in past). It displays a message like:
W: Failed to mount '/dev/sr0' to '/media/apt/'
E: Unable to locate any package files, perhaps this is not a Debian Disc or the wrong architecture?
I fount this workaround useful:
After doing dv3500ea's method's first two steps, (I write this again for user's convenience)
sudo mkdir /aptoncd-mountpoint
sudo mount /media/USB/aptoncd.iso /aptoncd-mountpoint -o loop
you should run this:
sudo ln -s /aptoncd-mountpoint /media/apt
If it gives you error, saying
/media/apt does not exists, create one with this
sudo mkdir /media/apt
Then run the third command of dv5300ea's command(slightly changed):
sudo apt-cdrom add
This should perfectly work.