The Elasticache getting started guide shows how to create a cluster
and connect to nodes in that cluster.
http://docs.amazonwebservices.com/AmazonElastiCache/latest/GettingStartedGuide/
From there you just need to configure hibernate-memcached to use the
nodes in your cluster
http://code.google.com/p/hibernate-memcached/wiki/Configuration
hibernate.memcached.servers - Space delimited list of memcached
instances in host:port format
As for playing with it, the getting started guide explains that you
can only connect to ElastiCache nodes from an EC2 instance in the same
security group.
So create a micro instance for that security group and SSH into it.
Then you can do stuff.
I was able to find these answers via Google, do some experimenting and
you'll get it to work I'm sure.