draw that top shape

How to draw that rounded shape (best picture I could find) in solidoworks? I´ve tried different methods, ->picture of an example box
2006-02-08_102209_forum_shape.rar

just the top shape that is interesting! I could´nt get that smoth ball shape on top of a box.

difficult to see on that picture, But I start with a plane with a rectangel and extrudes it, but I want one of the two large sides to be rounded, not flat!