Were you using the SAME Oxygene version and did you drop it on the Designer window? If so, I wonder why mine doesn’t work. And it didn’t work on Win7 with an older Silverlight version and an older VS with an older Oxygene.
At least now, I get a message. Before it added the references, then deleted them. Automatically. 
So what is the next step. I can’t make this work. Any clues or ideas to try?
… time passes …
I just tried again with a Silverlight Web Application project and I tried dragging the ValidationSummary into the Grid in the Xaml. Now I get:
Error 1 The name "ValidationSummary" does not exist in the namespace "http://schemas.microsoft.com/client/2007". C:\Users\Mark\Documents\Visual Studio 2013\Projects\crap\SilverlightApplication31\SilverlightApplication31\Page.xaml 11 1 SilverlightApplication31
and
Error 2 The type 'ValidationSummary' was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built. C:\Users\Mark\Documents\Visual Studio 2013\Projects\crap\SilverlightApplication31\SilverlightApplication31\Page.xaml 11 2 SilverlightApplication31
Drag and dropping it in the Designer still gets the same error as before.
Luckily I can get around this by manually adding the references, and the namespace, and then typing the xaml in the editor, but it would be nice to be able to use drag and drop again. I haven’t been able to use it on two boxes with two versions of VS and two versions of Oxygene. Well, I’m not sure it ever worked right for me if the control came from an assembly not in the references.
FWIW, I tried the same sort of thing in WPF. I added an item that required it to add a namespace to the xaml and add a reference. All worked fine and dandy.
I wonder if you were using SL 5? No idea what the difference might be because I think this eliminates VS 2013 as the issue.
… time passes …
I just noticed that, after I added the ValidationSummary object to the toolbox, its icon is red and there is an exclamation mark on it. That sounds like some sort of problem which might be why the drag and drop is failing. Or is it as I originally thought, just reflecting the kind of red that the ValidationSummary presents in the UI?
… time passes …
I was able to successfully add an AccordionButton that required another assembly and all. Looked normal in the toolbox and added everything just fine. Maybe there is something specific about the ValidationSummary that causes the problem on my box.
DescriptionViewer presents the same problem and its icon is not red.
One difference between the DescriptionViewer and the ValidationSummary is that the ValidationSummary namespace is xmlns:sdk=“http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk” which it put in for me IF I put the required assembly in the references manually ahead of time.
I wonder if it is some problem with that “sdk” thingee that is really a collection of assemblies, as I understand it.