Networks Airport Express / DLink DSL604+ prob

Hi peeps, it's been a long time since i've posted on these boards, not got the spare cash to keep upgrading anymore! I've got a bit of a prob, wondering if anyone can help:

I want to use my newly purchased Airport Express in order to stream music from iTuens on my PC to it so I can listen to my tunes in the lounge. I've also got an XBox at the same location and wouldlike to use the ethernet port on the airport express to access XBox Live. the problem is, my desktop PC being used as the iTunes server is connected via 100Mbit LAN to my DLink DSL604+ wireless adl modem (through one of its 4 lan ports) but no matter what i try i cant get the airport express to connect thru wireless. i can plug it in via ethernet to set it up, but it wont connect wirelessly. help!

Many many thanx in advance to anyone who can help.