From the Admin Guide
http://www.vmware.com/pdf/vdr_10_admin.pdf
In the Credentials page, enter a username and password and click Next. Data Recovery uses this information to connect to vCenter to perform backups, so the specified user
account must have administrative rights.
If you are connecting via VDR vSphere Client plug-in, then It is asking for a password valid for vCenter Server. Usually the same login/password that you use to open vSphere Client will also work here. Most, if not all of what you can do with VDR is accessible from the VDR vSphere Client plug-in
There is a default login and password for the appliance (someone responded to this earlier) - this is only needed if you are accessing the appliance directly using a console connection.
In summary, use the same username and password that you use to access vCenter Server.