The page that loads the Silverlight is actually the app.aspx page located in the Web folder in the source code package. I may have renamed it on the demo site to Index.aspx.
Although the mojoportal-plus-silverlight.sln has a mojoPortal.Silverlight.Web project, that is really not used for integration with mojoPortal. That project simply has a post build event to copy the .xap to the Web/ClientBin and to copy the needed System.Web.Silverlight.dll to the Web/bin folder.
If you are trying to learn how to authenticate from Silverlight against the built in web services, the code of interest is in the mojoPortal.Silverlight.Framework project, especially in the ServiceProxies folder, as well as in Controls/SignInController.cs and SignInControl.xaml
Hope it helps,