well, the line you wrote implied you'd have installed the source packages and not the compiled ones and then compiled kde.
usually if it's only a single plasmoid then all that is useless and you should just download the plasmoid, untar, configure and make.
this page that refers to the weather plasmoid explains the passages to build a single plasmoid.
http://www.kde-look.org/content/show.php/Weather+Plasmoid?content=84251
there are the commands that need to be run to install it and the order to do so. just replace the name of the tar archive with the one of your archive.