Posted this question few days ago and got excellent replies. However, they didn't work because I can't get the ball out. Moving the battery compartment cover doesn't have any effect on the ball. Still stuck and still sticking!
Usually if you turn your mouse over, there is a circle around the mouse ball that can be rotated by hand. When the cicle is aligned correctly, it will drop out along with the mouse ball. To replace, align the correct part of the circle with the notches on the main body of the mouse. Sorry if you have explained that your mouse wasn't standard in your last post, in which case the above may not work. BTW if you have an optical mouse, the ball may not be designed to come out.