To download the floor plan, first add it to a presentation sheet and then download the same.

Reference articles,

How to add a floor plan to your presentation sheet?

http://help.infurnia.com/en/articles/8504677-how-to-add-a-floor-plan-to-your-presentation-sheet

How to download your presentation sheets?

http://help.infurnia.com/en/articles/8504706-how-to-download-your-presentation-sheets