Here are the Blend steps to how to add a RadDropDownButton and add a RadGridView as its content.
First I opened blend and created a new silverlight project.
Then from the Assets tab I found the RadDropDownButton.
I dragged a RadDropDownButton on the drawing canvas.
I selected the button and added a GridView as its content, using drag and drop.
This is how my drawing canvas looked like.
Then, to wire the SelectionChanged event, I selected the GridView and opened the Events tab (upper right corner).
All the events were there, including the SelectionChanged event.
Double-clicking the field created a SelectionChanged event handler.
Let me know if this helps or how I can be of further assistance.
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