There are different networking protocols in LAN technologies. One of them is the famous TCP/IP which runs the Internet. I'm not familar with MyBook technology, but I'm guessing they use the protocol (the Windows File Sharing stuff) that can work without TCP/IP.

But it seems silly to not support TCP/IP. If you have a router your that Ubuntu and the MyBook are plugged into, you can try looking there for any "DHCP leases" it's handed out. Alternatively try browsing the PUBLIC folder and then running the Terminal command "netstat". There be a lot, but any open TCP/IP connections will be listed at the top if you scroll up.

Finally, while I've never paid attention to MyBook before, there seems to be a sizable modding community and wiki. Of course all the documentation assumes you know the IP address.