Christmas present headache!

I am giving my mother a wi fi only ipad for Christmas and have also got her an Airport Express to provide a network. What I want to be able to do is to set up the Airport before I send her the whole package (becasue she is scared of technology) but I am having trouble doing this. She has a wifi enabled imac at home but does not have a wireless router. Every time I try and configure the Airport Express to just act as a wifi conduit from the imac it tells me that no ethernet cable is attached! I had an old Airport extreme which was configured to do just this thing but it seems that I can't set the new model up without connecting it to an ethernet cable.

If I were to connect and set Airport up via ethernet would her computer be able to read the configuration?