Install Smarty (and Pear):
aptitude install smarty php-pear php-db
Configure your PHP include path and open_basedir setting:
open_basedir = /var/www/example.com:/usr/share/php:/usr/share/php/smarty/libs include_path = ".:/var/www/example.com/inc:/usr/share/php:/usr/share/php/smarty/libs"