First you need to edit the /etc/apt/sources.list file

sudo gedit /etc/apt/sources.list

Add one of the following

deb http://ppa.launchpad.net/chromium-daily/ppa/ubuntu lucid main
deb-src http://ppa.launchpad.net/chromium-daily/ppa/ubuntu lucid main

Save and exit the file More...

$ sudo gedit /etc/apt/sources.list

deb http://ppa.launchpad.net/chromium-daily/ppa/ubuntu karmic main
deb-src http://ppa.launchpad.net/chromium-daily/ppa/ubuntu karmic main

$sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 0xfbef0d696de1c72ba5a835fe5a9bf3bb4e5e17b5

$sudo apt-get update

$sudo apt-get install chromium-browser