Actually I have to add some other component(s) to MenuBar at the right side i.e a Button and label and there will be some space in between them. And i have to align the Button to some other component ...
Like I said calling the set bounds after adding the child to the parent will fix your problem.
Setting the layout to null will give you immense grief if the applet is resized.
You might want to consider some other suitable layout (BorderLayout.EAST and BorderLayout.WEST for example) which would give you the same results and will take care of the resizing for you.