When you are using an iframe to display the widget it is important that you make the right choices otherwise your widget ends up looking terrible and/or you are left unbookable.
When using an iframe the best code to use can be found by logging into freetobook and going to :
directBookings/iFrame
You also need to ensure your widget is set to open in a new window (login and go to directBookings/bookingButtons/settings) otherwise it will try to open in a tiny frame.
If you are creating your site on wix.com then you should use the "Embed HTML" app from the "Add Apps" menu.