i belive there is a way to do it with java scripting, but a much easier way would be to:
make your Nav movie the size of your final product (800x600),'Load Movie' for your Main -'Stop' if you don't want it to loop, have your buttons 'Load Movie'
in your loaded movies you can specify their location with 'Set Property' _x, and/or _y="number of pixels offset from upper left of Nav"