This is something you'll have to take up with the Amanita Design support staff.
Please use the Support request form you'll find within the "
About" tab up there between "Games" and "Store". Also, be sure to compose a more specific Subject description than just "Install Error". A.D. support gets hundreds of emails a day; try to convey the seriousness of your problem in that Subject field so that they'll pay attention to you.
Good luck and PLEASE post a follow up when you get this sorted out.