I can't get the design mode to work in Flash Builder 4.0.1.
When I click the "design" button on the top of the Editor view it does indeed switch, but all new design-related views appear white, as if they were still loading. Clicking on each one separately makes them appear, but when the central one is clicked it still does not load and returns an error message. (the one where you would be supposed to lay down the interface)
Here's the error:
Clicking on the second line in the upper box shows the same error details.