Isolation Runner not working as expected?

Hi,
I would like to know if any one has used isolation runner for debugging?

I have tried a lot but it is not at all working?

My steps are as follows:
1. In the mapper function i have created an divided by zero exception.
2. In the hdfs-site.xml i have set the property keep.failed.task.files to true, in all the nodes
3. Submit the job.

Now each and every task are failing because of the Arithmetic Exceptions.
Now i went to the folder local/taskTracker/jobCache/task-id/work
inside the work i can see a tmp folder which is empty.

Now as per the documentation i ran the isolation runner:
bin/hadoop org.apache.hadoop.mapred.IsolationRunner ../job.xml

But it throws me class not found exception. So i checked the class path, and i think it is fine.

Can anyone help me?

Regards,
Sharma