The following error took me some time to resolve. It would have been useful if this were more actionable (even referencing which key pair it's talking about would have been useful). Eventually I figured out that must use a passwordless pair.
~/w/whirr-0.4.0-incubating$ bin/whirr launch-cluster --config ~/whirr-zk.properties
Exception in thread "main" org.apache.commons.configuration.ConfigurationException: Key pair is encrypted