Compile and install wmpus
git clone git://pileus.org/wmpus
cd wmpus
make
sudo make install
Download and install the session file to /etc/X11/Sessions/wmpus
mkdir -p /etc/X11/Sessions
cp wmpus.session /etc/X11/Sessions/wmpus
Download and install the desktop file to /usr/share/xsessions/wmpus.desktop
mkdir -p /usr/share/xsessions
cp wmpus.desktop /usr/share/xsessions/wmpus.desktop