How do I add the "proposed" repository?

From the wiki page on the -proposed repository:

To enable the proposed archive for Ubuntu 11.04 go to Applications→Ubuntu Software Center→Edit→Software Sources→Updates and ensure that natty-proposed is ticked.

Those directions map well to recent Ubuntu releases such as 10.10, 10.04, 9.10, etc.

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Here are recent pictures from 12.04.

Using the Command Line

Manual addition

With root permissions edit your /etc/apt/sources.list:

sudo vim /etc/apt/sources.list

Then look for the following line or add it yourself:

deb http://archive.ubuntu.com/ubuntu/ precise-proposed restricted main multiverse universe

(If you are not using precise, you can replace it with your current version)

Save your file. Then run sudo apt-get update and install the package you want to use sudo apt-get install package/precise-proposed or sudo apt-get -t precise-proposed install package.

One liners

sudo sh -c "echo 'deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe' >> /etc/apt/sources.list.d/proposed-repositories.list"
sudo apt-get update
sudo apt-get -t precise-proposed install some-package

Done.