To use a layout from a previous event you first need to save the layout as template.
In the seating tab, go to Floorplan tab on the left side menu.
Once you have set the layout, go to the template icon on the top:
Set a template name and save.
To use a saved template, you must be in the same room ( only you are using a venue floorplan).
Create a new floor plan, and go to the template icon, choose the template from the list: