To install plasmoid unpack archive, go to the directory
where this INSTALL file is and execute commands:

mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=`kde4-config --prefix`
make -j 2
sudo make install
kquitapp plasma-desktop && sleep 2 && plasma-desktop

Replace the "2" with the number of cores your computer has
to speed up the compile process (the runtime speed of the
compiled applet is unaffected by this).

On some installations the last line has to be this instead:
kquitapp plasma && sleep 2 && plasma

After that plasmoid should show up in add widgets dialog.