WD Mybook + airport express : Can't get it to work

Hi all, total noob here, throwing myself on the mercy of you all.

I’ve just bought a WD My book world edition 1tb (the white one) and simply cannot get it to work with my current set up. 

I’ve got a mac book pro & Maci mini.  Internet comes via an Airport Express plugged into a netgear adsl router dg834 v2 

I followed the set up instructions but neither computer can see the My Book.  However if I plug the Mac book into the router it can see it, ditto if I plug the My Book directly into the Mac Book’s either net port. 

So why can’t it see it over the airport, when the two computers can both see each other??

Confused by the whole thing, and any help or advice would be most greatfully recieved…

Much thanks


The device must be connected to a router not a modem

the airport express is a modem