...
Click on the "Save" button at the top of the page. Enter an explanation of what you did and "Save" to dismiss the dialog. Ambari will notifiy you that you need to restart Zeppelin for the change to take effect. Allow it to restart all impacted services. Once the restart is complete, you should be able to access Zeppelin on the master's IP address on port 9995.
Using the Cluster
You can interact with the cluster by ssh'ing into the master node as user centos.
To manage HDFS it's best to su to user hdfs and issue the management command line statements. I created an nds user and a home directory in hdfs.
You can directly interact with spark using the % spark-shell command