Connecting Airport Extreme to Existing Wifi Network


I have an existing Wi-Fi network in my office building. It is just a D-Link DSL-G604T that links to my ADSL connection and also acts as a switch for a network printer and a UPS backed data server.
We use Wi-Fi laptops - both Windoze and Mac in that building, but in my house nearby we have an iMac. The office building is close enough to the house to just get the D-Link signal if the iMac is at one side of the house.

The iMac connects to the D-Link and home office network/Internet fine... BUT

I want to move the iMac to part of the house that is out of range of the office's D-Link wireless router.
I thought I could use an Airport extreme base station we already have as a relay or remote base station for the 802.11b/g signal from the D-Link...

However, I am having real difficulties getting this to work.

Just while I'm setting up, I've disabled the access security - so the access is completely open, so it's not security issues.

I have the D-Link MAC address in the WDS part of the config, but cannot seem to get network or internet access..