So I follow the process religiously and end up with the classic and famous boring screen 'No bootable device - insert boot disk and press any key'. However, its optical drive does not work, so I chose the USB key solution using the trick of modifying the file ist of Bootcamp (type 'usb bootcamp youtube' in google to see a video of this trick).
I am trying to install Windows 7 on it via Bootcamp. I have an iMac (bought in 2011) and I recently upgraded it with Yosemite.