Maxtor Shared Storage (NAS) + Airport Express problems

Dear forum readers and network pundits,
I have just recently bought a 500Gb Maxtor Shared Storage (MSS) ethernet drive, to which I'm planning to make regular backup of files from my wirelessly connected MacBook Pro and Mac Mini. Both computers are connected to the Internet via Airport Express, which, in turn, is connected to a router/modem. The MSS is attached to an RJ45 socket on the modem, next to the Airport Express RJ45 cable.
The problem is that I cannot seem to make a proper connection to the MSS. I can access the drive's web interface through Safari, but I cannot mount it as a network drive in the Finder. When hooking up my MacBook Pro to the router/modem directly using a RJ45 cable everything works like a charm, so I know the drive works fine. The problem seems to be the transition from the wired router to Airport Express.
I'm no network wizard in any way. When things don't work from the start I'm left scratching my beard, which is why I turn to the vast bank of knowledge contained here. Please help me. Is there a button in the Airport Express settings I can push to make it work, or do I need to by more hardware?
Looking forward to your help.
P.S. I'll churn out brownie points by the bucketload for any helpful answers
MacBook Pro & MacMini   Mac OS X (10.4.7)  


This seems to me related to this problem
I think it's the mixture of wired and wireless in combination with certain routers.