How do I install playonlinux from the repository?

If you did not already do so, you will need to enable the Multiverse repository before trying to install playonlinux. If you did not do that already, that is why you can't find the playonlinux package with apt-get.

Please refer to this question and its answer which explains how to enable the multiverse repository.

For precise run the below commands to install playonlinux:

wget -q "http://deb.playonlinux.com/public.gpg" -O- | sudo apt-key add -
sudo wget http://deb.playonlinux.com/playonlinux_precise.list -O /etc/apt/sources.list.d/playonlinux.list
sudo apt-get update
sudo apt-get install playonlinux

Or

Download playonlinux.deb file from here

Note: For precise and above you must install the package wine:i386 to get PlayOnLinux working