I think you might not be setting the layout to appropriate container, try below code see if it suits your requirement.
Button topBar = new Button("TopBar");
List mylist = new List(new String[]{"Item 1","Item 2","Item 3"});
Button bottomBar = new Button("BottomBar");
Form form = new Form();
Container contentPane = form.getContentPane();
contentPane.setLayout(new BorderLayout());
contentPane.addComponent(BorderLayout.NORTH, topBar);
contentPane.addComponent(BorderLayout.CENTER, mylist);
contentPane.addComponent(BorderLayout.SOUTH, bottomBar);
contentPane.revalidate();
form.show();
The above code you can interchangeably use it with Container.
PS: In order to run the above code make sure you set the Resources and Theme correct in your code. If you need help on this check the LWUITDemo jar file / project that is included in the downloaded LWUIT library zip file.