To install plasmoid check if you have installed needed header files (kdelibs, kdebase-workspace), unpack archive and execute install.sh script.

Alternatively you can run following commands:

mkdir build
cd build
cmake ../ -DCMAKE_INSTALL_PREFIX=`kde4-config --prefix`
make
sudo make install
kbuildsycoca4

After that plasmoid should show up in add widgets dialog.

If not, you can try to execute:

kquitapp plasma-desktop
plasma-desktop
