First of all you need be sure that your Memcached configured and working properly. Check memcached port (11211 if default settings) / process etc... for example memcached -u www-data -vv. Then if you using memcached default configurations you should change core.php configurations like following: Uncomment section about memcached. After it it's should looks like this:
Cache::config('default', array(
'engine' => 'Memcache', //[required]
'duration' => 1800, //[optional]
'probability' => 100, //[optional]
'prefix' => Inflector::slug(APP_DIR) . '_',
'servers' => array(
'127.0.0.1:11211'),
'persistent' => true,
'compress' => false));
Now change $engine = 'File'; to $engine = 'Memcache';
Use caching for example in controller you need write data with key => value, then access that data with key. Example:
Cache::write($key, $value);
Cache::read($key);
That's all. Hope it's help you.