Hi,
The yellow border means that this is your insertion point. By default it will be on the LayoutRoot. If you add another control in to LayoutRoot such as a stack panel and you want to insert a button in to the new StackPanel - you must first double click
the stack panel to set it as the insertion target. This will give the StackPanel the yellow line.
You can then insert a button in to the StackPanel.
Hope you are enjoying Blend!
Cheers,
Jordan.