Editar archivo sources.list :

gksudo gedit /etc/apt/sources.list

agregar el repositorio:

deb http://repository.cairo-dock.org/ubuntu jaunty cairo-dock # For Ubuntu 9.04

deb http://repository.cairo-dock.org/ubuntu intrepid cairo-dock # For Ubuntu 8.10

deb http://repository.cairo-dock.org/ubuntu hardy cairo-dock # For ubuntu 8.04

GPG key :

wget -q http://repository.cairo-dock.org/cairo-dock.gpg -O- | sudo apt-key add -

instalar:

sudo apt-get update
sudo apt-get install cairo-dock cairo-dock-plug-ins

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Fuente: https://help.ubuntu.com/community/CairoDock