Designer version tested: Designer 2.0.0 build 309
Operating System: Mac OSX Lion
Framework: Ext JS 4.x

The designer does not include the buttons property: Ext.window.Window > buttons

You can add the buttons property manually to a Window with...

buttons: [{ text: 'Button 1' }]
but then the buttons need to be edited within the property value rather than designed (e.g. you can drag in buttons from the Toolbox, you can't edit their text by double clicking, you can't attach event bindings, etc.)