Type the following commands:

wget -q "http://deb.playonlinux.com/public.gpg" -O - | sudo apt-key add -
sudo wget http://deb.playonlinux.com/playonlinux_$(lsb_release -cs).list -O /etc/apt/sources.list.d/playonlinux.list
sudo apt-get update
sudo apt-get install playonlinux