Fedora 15 e gli MPEG2

[Alhazred]

I'm trying to install the codecs to play mp3 and divx, but I have some troubles.

Both RythmBox and the movie player tell me that the codecs are not installed, so I click on the button to search them, but they are not found.

I've installed some third party repository, inside them I can fiind the codec, but when I try to install them I get this message:

Recupero chiave GPG fallito: [Errno 14] Could not open/read file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmfusion-free-fedora-15-x86_64

it is in Italian, it sounds like "Can't get the GPG key"

I've tried
su -c 'yum update'

but the problem persists, what should I try?
I use Fedora15 64 bit