There is a forum just for the application builder at http://forums.arcgis.com/forums/211-ArcGIS-Viewer-for-Silverlight which might help.
But one problem you might be having if you downloaded the sample from http://help.arcgis.com/en/webapps/silverlightviewer/samples/start.html is that you have to do a build/debug on the silverlight solution in Visual Studio before you upload the xap file.