See if you can reach the NameNode server with your browser:
http://$namenode.full.hostname:50070
Try copying a file into HDFS and listing that file:
su $HDFS_USER /usr/lib/hadoop/sbin/hadoop dfs -copyFromLocal /etc/passwd passwd-test /usr/lib/hadoop/sbin/hadoop dfs -ls
Test browsing HDFS:
http://$datanode.full.hostname:50075/browseDirectory.jsp?namenodeInfoPort=50070&dir=/&nnaddr=$datanode.full.hostname:8020