Kali Linux Dual Boot - Wifi not working
I don't know what do you mean by where to find /etc/apt/sources.list . It's in /etc/apt/ as the path indicates. You can edit this from your terminal, I prefer using vim. sudo vim /etc/apt/sources.list, hit Shift-G to jump to the end of the file, followed by 'o' to go into insert mode on a new line and write deb http://http.debian.net/debian/ wheezy main contrib non-free. Then hit the escape key followed by  ':' then 'wq' and hit enter to save and quit. (you're free to use any editor you want just remember to run it as root with sudo)
Kali is based on Debian Wheezy: http://docs.kali.org/policy/kali-linux-relationship-with-debian
When the first step is done correctly you should be able to do what the debian wiki says.