It should just work. For example, the following works for me.
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.dom.client.Style.Unit;
import com.google.gwt.event.dom.client.*;
import com.google.gwt.user.client.ui.*;
public class Bar implements EntryPoint {
public void onModuleLoad() {
final DockLayoutPanel dock = new DockLayoutPanel(Unit.EM);
final FlexTable table = new FlexTable();
final FlowPanel header = new FlowPanel();
header.add(new Button("Add row", new ClickHandler() {
public void onClick(ClickEvent event) {
table.setText(table.getRowCount(), 0, "foo");
}
}));
dock.addNorth(header, 5);
dock.add(new ScrollPanel(table));
RootLayoutPanel.get().add(dock);