Installing the APC with PHP5.3 is straightforward (when you know how):
1. Install the required packages:
sudo apt-get install php-pear php5-dev apache2-threaded-dev
2. Install APC:
sudo pecl install apc-3.1.4
3. Create file /etc/php5/conf.d/apc.ini with the following content:
extension=apc.so
4. Restart Apache:
sudo /etc/init.d/apache2 restart
After restarting the Apache2 web server apc.ini section should be included in the phpinfo() output.