In order to access the latest disko packages, you have to add following line the sources.list.

deb http://www.diskohq.com/ubuntu/ karmic nightly

It can be done by modifying the /etc/apt/sources.list directly with your favorite editor or using following command.

sudo sh -c 'echo "\ndeb http://www.diskohq.com/ubuntu/ karmic nightly" \
  >> /etc/apt/sources.list'

But it might be more convenient for you if you create a new file in /etc/apt/sources.list.d/

sudo sh -c 'echo -e "deb http://www.diskohq.com/ubuntu/ karmic nightly" \
  > /etc/apt/sources.list.d/disko.list'

To authenticate the disko packages you have to add the public key to apt.

wget -qO- http://www.diskohq.com/disko.pkey | sudo apt-key add -

You might want to install the git version which is built regularly.

sudo apt-get install disko-git-dev