You have to add the pokerTH teams PPA.
You can get the PPA for ubuntu 12.10 from the games Launchpad site.
To add a PPA, open your terminal (CTRL+T or search the dash)
sudo apt-add-repository ppa:pkg-games/ppa
it will ask for your root password (admin) and you will not see anything as you type it in, but that's alright. After writing the password hit RETURN / ENTER
Then you will have to update all you software sources, by typing:
sudo apt-get update
and finally you can install the game
sudo apt-get install pokerth