ok i fixed it.
i downloaded version carddav_0.8.0.tar.bz2 from
http://www.crash-override.net/carddavdownload.html
extracted it to /plugins, renamed it to carddav and executed the init mysql script
then i found CARDAV field in settings!!
it didnt work with the version that i downloaded with plugin_manager at all!
i didnt have the CARDAV field in settings and it didnt synced.
why not?
MyRoundCube is not part of the RoundCube project, it would be better if you ask them for support.