SceneBuilder

Scene Builder is a visual, drag'n'drop, layout tool for designing JavaFX application user interfaces.

SceneBuilder view

It can be used to modify or extend the SimStadt GUI, and the corresponding .fxml files located in:

building-energy-libs/src/main/resources/eu/simstadt/library/editor/
hierarchic-workflow/src/main/resources/de/hftstuttgart/hierarchicworkflow/gui/
simstadt/src/main/resources/eu/simstadt/

Open with SceneBuilder

Eclipse plugin

e(fx)clipse is an Eclipse plugin which might help working with fxml files.