There's no way to get that specific. You can go into Start > Control Panel >
Mouse > Motion and check the box for "Move pointer to the default button in
dialog boxes", but that will do the same in every dialog in all
applications. Maybe that's what you really want...
If the OK button is the "default" button (shown by a thicker black border
than the others), pressing Enter will have the same effect as clicking the
button with the mouse.