Looks like there was a bug fixed https://issues.apache.org/jira/browse/AMBARI-3792
I suggest first upgrading to Ambari 1.6.1 (which is the latest ambari release), then once on 1.6.1 add datanodes/hosts:
http://docs.hortonworks.com/HDPDocuments/Ambari-1.6.1.0/index.html