NOTE: before enabling cache make sure you have installed and enabled the cache extension.
You need to edit your config.php file and the following line
You need to know ip and port of the machine/s where memcached service is installed and then add the following lines to the file config.php
$_cache_config = array(
'default_host' => '127.0.0.1',
'default_port' => 11211,
'default_weight' => 1