Problem with Airport Express

Hi,
I'm having a problem with Airport express. I don't know if what I'm trying to do is possible and I'm only getting confused trying to figure it out myself.
I have an existing cable modem connection. It goes to a router and then on to three other rooms in the house. What I want to do with the airport is have it take the connection from one of those three rooms and send it to another room wirelessly. (For odd reasons, a hardwired connection isn't possible in the target room.)
I've plugged in the Airport, followed the instructions that came with it, and wasn't able to get it working at all. Is there a reference (outside of the standard help on Apple's website) that anyone can point me to? Or maybe walk me through it on this forum. I'm initially stuck on whether it should be considered a WDS or not.
I went to the Apple support website, and I can't tell if I have free phone support or not, although I know they offer it for some products. The papers that came with the Airport don't say anything about it, so far as I've seen.
Any help would be very much appreciated.

Thanks a lot,

Phillip

 

I take it you want to wire it to another router, and use it as an access point, right?

Yes, this is what I'm trying to do. I initially set it up as you recommended after reading the setup guide, but it didn't work. I tried resetting it a few times, but it still didn't work. This is why I was wondering about other possibilities in confusion - WDS, etc.

If it's supposed to work like this, the only question I can think of is about the DHCP option for getting an IP address. I have the option for "distributing IP addresses" deselected, but should DHCP somehow be deselected in getting an IP address for the airport? I tried selecting "manually," but obviously that means I need to enter a # in, and I don't know what to enter.

Thanks for your help.

with best wishes,
Phillip

p.s. I'm using a Mac Mini - so compatibility shouldn't be a problem