I have the same problem (using Windows Vista) and none of the suggestions above have helped to get it mapped. It seems not to be a firewall (Norton 360 Windows firewall turned off, ) or security issue.
when I enter the IP address or server name into the address bar of Internet Explorer I get the error message:
Windows cannot find '\'. Check the spelling and try again.
When I try to open the Network with the shortcut generated by WD Discovery I get the error message:
\ is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.
The specified network provider name is invalid.
Additional information, both my laptop and MyBookWorld is on the same workgroup.