1. go to zend page and download the optimizer.
2. then unzip it on your hdd.
3. copy the file corresponding file to your php version (ZendOptimizer.so) to /usr/local/lib/Zend/ (create the directory if needed).
4. add in /etc/php4/apache2/php.ini this at the beginning after [PHP]
zend_extension=/usr/local/lib/Zend/ZendOptimizer.so
zend_optimizer.optimization_level=15
5. restart apache (/etc/init.d/apache2 restart)
6. check you results (php info)