First off, you'll need the following:
- A running memcached server, and the address it is running at
- The PHP memcached extension, installed on the server running MODX
Setting up Memcache in MODX
Go to System Settings, and change the "cache_handler" system setting to "cache.xPDOMemCache".
If you have more than one MODX site on your server with cache.xPDOMemCache cache handler, you need to create new system setting "cache_prefix" with a value like "yoursite_" to differentiate cache keys for different sites.
If your memcached server is on a separate server, you can set the path to it with the System Setting "resource_memcached_server", with a value like "memcache.tld:121212"