In blend 4 i often open a resource dictionary (i.e. "Generic.xaml") by clicking the "resources" tab (next to the property tab on the right pane), navigate to a resource and click on its icon to edit it in the designer.
In blend for visual studio 2012 i can't seem to find the resources tab? How am i then able to edit resources in resource dictionaries?
I'm assuming this is a XAML project. Is the Window open? Main toolbar -> Window -> Resources... is that checked? If not, check that.