Determine if you can reach the NameNode server with your browser:

http://$namenode.full.hostname:50070

Create the hdfs user directory in HDFS:

su - $HDFS_USER
hdfs dfs -mkdir -p /user/hdfs

Try copying a file into HDFS and listing that file:

su - $HDFS_USER
hdfs dfs -copyFromLocal /etc/passwd passwd 
hdfs dfs -ls

Use the Namenode web UI and the Utilities menu to browse the file system.