Windows installation problem

Hey there people
So, I have a MacBook Pro(Late 2011) 13 inch running OSX Yosemite 10.10.2 and I wanted to mount Windows 8 professional via bootcamp with usb drive. I've already watched a ton of tutorials and read a whole **** of stuff, but I see only one difference in my problem so far. So, I have a 16gb usb drive, a windows iso file and all the rest of the stuff required. But the only thing that I meet so far is that when everybody else I've seen doing this open Bootcamp Assistant, they have three choices right away, Yet, what I have is only 2 choice variants and my bootcamp assistant for some reason lacks the "create a Windows 7 or later install disk" choice button. ***? Could anybody please tell me what the **** and how can I mount windows in this case or what am I supposed to do or whatever? Really need it guys, HELP!