Thanks in advance.
put these lines in config/site.php and concrete5 should be looking for a memcached server on localhost.
there are two constants CACHE_BACKEND_OPTIONS and CACHE_FRONTEND_OPTIONS which you can use to serialize Zend Cache settings. Check this page if you need for information about the options you can specify:http://framework.zend.com/manual/1.12/en/zend.cache.backends.html...
But keep in mind that Zend Cache has less use in 5.6.1 (the upcoming release of concrete5). It might be a waste of time to look into memcached as some of my testsites run a lot better with 5.6.1.