Install Windows 7 with Bootcamp on OS X Yosemite with USB Key

I have an iMac (bought in 2011) and I recently upgraded it with Yosemite. I am trying to install Windows 7 on it via Bootcamp.

However, its optical drive does not work, so I chose the USB key solution using the trick of modifying the file info.plist of Bootcamp (type 'usb bootcamp youtube' in google to see a video of this trick).

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'.

So I tried to use Unetbootin ( http://ift.tt/jSIdcH ) to make my USB Key bootable, and tried to boot on it through the screen that appears when you maintain the Option key (Alt button) while starting the mac. Result: I never managed to have my Unetbootin-formatted USB key appear on this screen :(

I read some stuff about reFit ( http://ift.tt/PZWUbf ) and GParted, but I just don't know how to use them...

If someone managed to install Windows 7 on his/her iMac with Yosemite using a USB Key, please help me understand why I can't see the USB as a boot option.