How to log messages from Hadoop?
If you go to your cluster's job tracker, then to your individual jobs you should get a list of tasks (maps and reduces) that were executed then there should be a link on the right to view the logs for that task. You can then see the stdout, syserr, and syslogs there I think. Check out stdout (not all of them will have an output but at least one will in my experience...)
Good luck!