well i dont really know anything about yum.
yes that's a shame.
yes i'm ashamed.
but ive read somewhere something like "yum install xxxxxx".
i didnt know if it worked with .tar files. i guess not. so i tried.
here's what i did :

[link@localhost downloads]$ su
Password:
[root@localhost downloads]# yum install xine-lib-1.0.tar.gz

You have enabled checking of packages via GPG keys. This is a good thing.
However, you do not have any GPG public keys installed. You need to download
the keys for packages you wish to install and install them.
You can do that by running the command:
rpm --import public.gpg.key
For more information contact your distribution or package provider.
[root@localhost downloads]# rpm --import public.gpg.key
erreur: public.gpg.key: échec de la lecture d'import.
[root@localhost downloads]#

duh
what's the problem?

With "xxxxxx" being an RPM package name of a package provided in one of the repositories Yum has been configured to access.
Yum is for installing RPM packages, not tar.gz archives.
The error told you to download a repository's public GPG key. You didn't do that, but instead typed in an example only.

With Fedora Core 3, run

rpm --import /usr/share/doc/fedora-release-3/RPM-GPG-KEY-fedora

To enable other Yum package repositories, you need to add/edit files in the /etc/yum.repos.d/ directory. E.g. Fedora Extras http://download.fedora.redhat.com/pu.../extras/EXTRAS
or http://rpm.livna.org/configuration.html (for things like mp3 support and movie players).