After a long time of refactoring, I've finally put up the third pre-alpha release of the SharpDevelop Class Diagram Addin.
Most of the changes were done internally in the architecture, and this work is not yet done. However, you will see the new placeholder for the Class Editor, that will allow you one day to edit your classes structures using a simple grid view.
Right for now, selecting a class in the Class Diagram view will set that class in the Class Editor Pad, and double clicking a member will bring you to it in the code editing window (nothing different than the Class Browser Pad).
Well, until next release, enjoy the new features, and report severe bugs ;)