The maximum zoom level in a MacDraw Pro document of one page is 3200%. However, because MacDraw Pro decreases the maximum zoom level based upon the number of pages in the drawing area, if you use the largest possible Drawing Size, the maximum Zoom level will be 400%. When you have reached the maximum zoom level, the zoom-in mountain icon will be dimmed. This is a limitation of MacDraw Pro.
The best solution is to upgrade to ClarisDraw, which does not determine the maximum Zoom level based on the size of the drawing area. In ClarisDraw, you can use any zoom level at any time.