Do you want to display all canvases at a time. If you dont want the stacked canvas to appear on tab canvas positioning your canvas correctly will solve your problem. I am wondering why you are not able to do it yourself.
If we could actually see your form we could help you better i think.
First of all: be sure that your window property is not null. If so, then navigate to existing canvas/window you want to display your stacked canvas and select from menu: View, Stacked View option. Then you'll be able to show/edit canvas within Forms Builder. If you're unable to see stacked canvas ad runtime then be sure that your viewport properties are set correctly(e.g. they are places out off parent window and canvas)