This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
creating a new interface class and then chosing open with window builder.
Disclaimer: I prefer to hand code and haven't used the window builder.
I doubt if you will be able to edit an interface with the window builder. Try creating a Composite or subclass a Widget and then editing with the window builder