PackageKit is the default package management system on Fedora from versions 9, 10, 11 and up. Package kit has great many advantages, read about them here. But for all its merits, it has one tiny niggle. Sometimes, especially on new OS installs, or after adding software sources, it fails to automatically refresh its cache. And the GUI (currently) has no way of doing that for you. So you will have to show it some good old command line love.

When PackageKit fails to refresh the cache, you either get an error that reads "No results were found. Try entering a package name in the search bar." for every category of packages, or the packages you expect to show up, from the newly added software sources, don't show up.
Graphically:

Refreshing the cache from the command line is simple. The text mode program that allows you to talk to PackageKit via the command line is "pkcon". To refresh the cache type this at the prompt:

$ pkcon refresh

pkcon will run through the refresh routine and PackageKit should work as expected.
Graphically:

If you wish the explore pkcon a little more, type this:

$ pkcon -?

into the command prompt to print the pkcon help.

You might also like to learn that pkcon can not be run as a root user. GTK+ applications should not be run as root. Doing that blows huge security holes into your system. Read more here. If you are in the mood for a little more reading you might want to look into this interview on fedoraproject.org. It talks about the rationale behind the inclusion of Packagekit into Fedora, its advantages and future goals.