package management - How do I add the "proposed" repository?

I'm trying to either test a fix that's been proposed or I want to volunteer to test stable release updates before they are pushed out to general users, how can I enable 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.