Since no one replied I'll jump in and maybe someone will get you more help. As a disclaimer: I don't have an Airport Express but do have a MBP. Anyway, if you can't get it to work you know you can share you connection through your airport card right?
Mac OS X 10.5 Help: Sharing your Internet connection
The article seems pretty straightforward. From the comments it sounds as if you use airport express as a device on the network as opposed to being your main router. From what I gather from the comments, if you set up the express as a router at your house, when you get to disney it will be plug and play. However if you set it at your house to stream music you will have to follow the guide's directions.
Edit: I just reread. You said you use it as your main router, so disregard that part.