You should try to add these credentials to the core-site.xml file. You can add the s3 credentials manually in the nodes or by using the boostrap action while launching the cluster.
You can launch the cluster with something like this:
ruby elastic-mapreduce --create --alive --plain-output --master-instance-type m1.xlarge --slave-instance-type m1.xlarge --num-instances 11 --name "My Super Cluster" --bootstrap-action s3://elasticmapreduce/bootstrap-actions/configure-hadoop --args -c,fs.s3.awsAccessKeyId=< access-key >,-c,fs.s3.awsSecretAccessKey=< secret-key >
This should override the default values which are placed by EMR as per the account launching the cluster.