Can't use wireless USB broadband while connected to airport express
I have a Sprint wireless broadband card, which I can connect to the internet just fine as long as I'm not connected to my airport express. I have an airport express that is hooked to my printer and stereo so I print wirelessly and listen to my iTunes thru my stereo, but it is not hooked to the internet in any way, and it works good for this. The problem is if I connect to to my airport express my wireless broadband will not work.
Your wireless network setup is not optimal. I suspect you have TWO wireless networks. Since your computer likely only has ONE wireless network card, it can only connect to one at a time. You need to have your Express +Join an existing wireless network+ created by your broadband. You'll need to provide more info on your broadband card if you cannot set the Express up properly.
My broad band card is connected to my computer thru the USB port. I might not be using all the proper terms. My broadband card is wireless in that it connects to the internet via cell phone signals, but is physically connected to the computer via the USB port.
The problem is that your Mac is only expecting to use a single network connection at any given time. Therefore you must decide if you want to connect via USB or AirPort.