Hello fellas!
i'm looking for the plugin "Blue Screen", to work with chroma key. but my kdenlive dosn't have this plugin. how to install it? thanks!

I'm afraid that without any further information about where you got your Kdenlive package from and which distro you are talking about, none will be able to help you. There is no such thing as a single Kdenlive installation. Please make sure that you have installed a recent version of Kdenlive suitable for your distro, together with the corresponding frei0r and mlt packages. Unless you happen to work with some minor and totally messed up distro you should have the chroma key effect readily installed as part of your Kdenlive installation. It's part of the same overall package.

For details about how to use chroma keying in Kdenlive, please head over to http://userbase.kde.org/Kdenlive/Manual/Effects/Alpha_manipulation/Blue_Screen; this should explain the details and also help you to find the correct effect in your installation.