Graph graphControl = viewer.getGraphControl();
DropTarget dropTarget = new DropTarget(graphControl, DND.DROP_MOVE | DND.DROP_COPY);
dropTarget.setTransfer(new Transfer[] { TextTransfer.getInstance() }); // or your Transfer
dropTarget.addDropListener(new DropTargetListener() { /*...*/ });

				viewer.addDropSupport(DND.DROP_LINK,
				new Transfer[] { LocalTransfer.getInstance() },
				new ViewerDropAdapter(viewer) {

					@Override
					public boolean validateDrop(Object target, int operation,
							TransferData transferType) {
						if (target != null) {
							System.out.println(target);
						}
						
						return false;
					}

					@Override
					public boolean performDrop(Object data) {
						return true;
					}

					@Override
					protected Object determineTarget(DropTargetEvent event) {
						Graph g = viewer.getGraphControl();
						Point p = g.getDisplay().map(g.getShell(), g, event.x,
								event.y);
						IFigure fig = g.getFigureAt(p.x, p.y);
						return fig;
					}
				});

@Override
protected Object determineTarget(DropTargetEvent event)
{
    Graph g = ((GraphViewer) getViewer()).getGraphControl();
    Point p = g.getDisplay().map(null, g, event.x, event.y);
    IFigure fig = g.getFigureAt(p.x, p.y);
    return fig;
 }