how to install source packages?

[Jud Craft <craftjml gmail com>]

I've enabled the updates-source and fedora-source repositories in
PackageKit, but how do I, for example, install the source package for
Cairo?

A packagekit search for Cairo returns nothing from the updates-source
or fedora-source repositories, and nothing called "cairo-source" or
anything of the like.  Just binary cairo and cairo-devel.

In short, how do I install source packages?  Have I missed an obvious
step or is it really unintuitive?