I think that you would have a page open, than click the "Add Viewport" button, than click out into the page. maybe it would create a standard size viewport that you can than change, or maybe when you clicked, it would start a corner of a rectangle, than you would drag/click to end the rectangle and that would be the viewport.
Or, you could draw a rectangle, right-click on it, and in the drop down there could be a "Make Viewport" choice.
(crojack)
