Ping is disabled on all appliances by default you will have to enable that setting to be able to ping. You must add this device to the ESM and configure ping from the GUI:
ADM Properties> ADM Configuration> Interface> Advanced> Enable PING
To answer your second question,
Your root password for this device will be set when you pair it to the ESM.
One side note - you can change the CLI password once you are logged in to the CLI - this only changes the local root password, and not the Keyed password.
If / when you Re-Key the appliance, it will also change the root password as killerkitty noted above.
This can get confusing if you change the ESM's CLI password once logged in to the CLI, you then have a different password to log in to the UI as NGCP than if you ssh in to the ESM as root.
I am unable to add ADM as data source in ESM,So how can i enable ping for ADM through ESM.
ADM is a Device, not a Data Source.
You add it like you would add a Receiver, ELM, ACE, etc...
From the top level Physical Display click on the "Add Device" icon
Then select "McAfee Application Data Monitor"