OK. I understand now that you have modified the kit to make _Default derive from MasterPage rather than Page. That's good. The errors you are seeing are coming from deep within the framework. They have to do with the fact that earlier _Default was declared to be derived from a different class. One way to get rid of those errors is to delete the temp files that the ASP.NET framework creates when it compiles the pages for a site.
You need to shut down Cassini if it is running (i.e., the web server normally used when you run from Visual Studio) and any sites from IIS that might be running on the box. Then you need to go to
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files
There, you'll see a bunch of folders that the framework has created. They need to be deleted. If you are told that some are locked and can't be deleted then you haven't shut down your web servers sufficiently.
After you get those files deleted the errors you show directly above in this post should go away.
Russ Helfand
Groovybits.com