PS3 need DLNA Server to connect to your media via LAN
Also your MyBook do not have any DLNA Server (mediatomb, twonkyvision, …) installed on it.
You can install one manually if you have a little experience on Linux.
First i tested a Orb 2.0 (Free) installed on Windows PC with media shared on MyBook through network.
It works, but navigation on PS3 is very slow, and DivX don't realy work (very very very slowly) because Orb make RealTime recompression on medias.
Secondly i tested twonkyvision (not Free, but 30 day trial) directly installed on my MyBook, following tutorials and help on this super forum :) ,and it work very perfectly on my PS3, video audio pictures everything is perfect.
(sorry for my English, i'm French - but i hope you understand me :)