Boot Camp "Install Disk missing" error

Hi,

I am trying to install a dual boot windows 8.1/El Capitan on my macbook pro 13' (2011) using Boot Camp 6.0.0. I have created a bootable USB key from an iso using UNetbootin. After the download of windows drivers for mac by Boot Camp, I have an error saying "Install Disk missing" whereas USB key is connected and recognize by disk utilities and finder.

Does anyone can help me ?

Sorry for my bad english, i am french.