First of all i would like say sorry to everibody if i used this forum in a wrong way but as a newbie an everithing i did not know some thinghs.
I hope to be better in the future.

My gmplayer says there are no skins in the directories /root/.mplayer/Skin and /usr/share/mplayer/Skin/default.
What should i do?
I added all the skins i own in this two directories, i tried to edit the file gui.conf but i don't know syntax. tried to install skins with rpm -ivh. No success!

SKIN dir 1: '/root/.mplayer/Skin'
SKIN dir 2: '/usr/local/share/mplayer/Skin'
[skin] file ( /usr/local/share/mplayer/Skin/default/skin ) not found.
Skin not found ( default ).

What am i missing?

You need to add a skin to that directory (in it's own folder) and call that folder "default".

For example - if you download the skin "Blue". Copy the "Blue" Folder to /usr/local/share/mplayer/Skin/ and rename it to default. It should now read: /usr/local/share/mplayer/Skin/default/

I did the same thing.

I added all my skin folder as you said, but probably the problem is that the skin is .rpm. could this be a problem?
Now i have a file called mplayer-skin-gnome-1.1-2.noarch.rpm
already installed with rpm -ivh in /usr/local/share/mplayer/Skin/default.
Is it ok?
I don't think so because gmplayer doesn't run.

If you installed the rpm with rpm -i rpmname.rpm then it should've put all the necesary files in the mplayer skin directory.
If you have a rpm-file in the mplayer skins directory you did something wrong. The skin files should be in a directory.

You could install the newest version from mplayer. If i'm not mistaken the default behaviour has been changed. If the skin you configured does not exist it will revert to the default skin.
At that point you can use the skinbrowser to choose your new skin.