Cannot add controls to mobile flex application in design view

I am not able to add controls to a mobile flex mobile project with view-based navigation.  When I try to drop the controls on the design, I get a not allowed logo over the entire area.  I am not sure what I am doing wrong.  If I add the controls in source view, I will be able to see them when I run the debugger but not when I switch to source view.  Any help would be greatly appreciated.