The easiest/quickest way to fix a cross domain error is to create a test website to host your Silverlight application.
When you hit F5 to debug the application, it will run the development server and the address bar will say http://localhost:xxx/blahblah. This will prevent a cross-domain error.
If you try to run it using an test html page from the file system i.e. browser address would say something like c:projectssilverlighttest.html then it will throw the cross-domain error.
If you already have your Silverlight application created then just create an empty website or web application project and then right-click on it and click "Add Silverlight Link…"
Then follow the instructions on the screen