If you want to add EC2 nodes as your slave nodes of existing Hadoop cluster:
Step1] Create a EC2 instance which has hadoop installed.
ec2-describe-images -x all | grep hadoop
Step2] Make sure that the version of hadoop MUST be identical on all the machines in your cluster.
Step3] Generate public key on your master machine:
ssh-keygen -t rsa
Then your publickey is stored in,
Step4] Copy your public key (id_rsa.pub) to .ssh/authorized_keys on your EC2 instance. Now you can ssh to your instance without your keypair.
Step5] Add your new slave node to your hadoop_loc/conf/slaves
Mine looks like:
Step6] Synchronize or copy your configufation files to the slave nodes:
Step7] format your namenode (if you need)
Step8] start your cluster!!