If you can’t attach the debugger to a Silverlight application opened in new browser window using HtmlPage.Window.Navigate method then do the following:

1. Go to Debug –> Attach to Process
2. In Attach to Process dialog select process which host Silverlight application
3. Click on “Select…” button under “Attach to:” section
4. In Select Code Type window select “Debug this code type” option and uncheck all checkboxes except Silverlight
5. Click OK –> Attach