AspDotNetStorefrontImportWebServiceWse is an added section that references Microsoft Web Services 3.0 in order to use the Username Token Management for security purposes. The reference.cs file that you are looking for, can be found in the WSI test application that you can download at http://manual.aspdotnetstorefront.com/wsi.
Alternatively, adding a web reference to the web service URL will create the reference.cs file for you, but again it will be missing the AspDotNetStorefrontImportWebServiceWse section. You'll have to retrieve that from the reference.cs file from the test application download and merge that section into the one created.
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>