You need to install CURL support for php.
In Ubuntu you can install it via
sudo apt-get install php5-curl
If you’re using apt-get then you won’t need to edit any PHP configuration, but you will need to restart your Apache.
sudo /etc/init.d/apache2 restart
If you’re still getting issues, then try and use phpinfo() to make sure that CURL is listed as installed. (If it isn’t, then you may need to open another question, asking why your packages aren’t installing.)
There is an installation manual in the PHP CURL documentation.