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. Login as
$HDFS_USER
:/usr/lib/hadoop/bin/hadoop dfs -copyFromLocal /etc/passwd passwd-test /usr/lib/hadoop/bin/hadoop dfs -ls
Test browsing HDFS:
http://$datanode.full.hostname:50075/browseDirectory.jsp?dir=/