Q: Product codes, bootcamp and WININSTALL?

So I just went through bootcamp to download windows for my mac and after it restarted my computer I got a question asking for my product code. However I don't have one. So I clicked on the 'I don't have a product code' and it lead to me to a part where I had to choose where the windows would go. But the only choices began with Disc 0 (or something along those lines) and my mac said that I couldn't put it on anything with disc 0 in it. So I closed the windows part and restarted my mac as normal. So now I have my normal OS X running and I have a WININSTALL file on my desktop and I don't know what to do with it and how I can get windows running on it. Do I need a product code? Do I need to buy one? Do I need to start bootcamp all again and reinstall windows or can I go through the WININSTALL file?

If you need any information on my mac then it's MacBook Pro (Retina, 15-inch, Mid 2015) and on Version 10.10.5. The processor is 2.2 GHz Intel Core i7. (I don't know if that is useful or not)

Many thanks,

Sara