The IP is assigned by the router.
The default user name and password will depend on the model drive you have.
If you have the My Book World Blue Rings try the following article:
How to reset the username, password, and network settings of a My Book World Edition (Blue Rings)
http://wdc.custhelp.com/app/answers/detail/a_id/1521
If on the other hand you have the MBW White Light, check the following:
Resetting a WD My Book World (white light) to the default username, password, and IP settings
