It should work via this link. If it doesn't, just let me know!
Are you on the Kindle listing? For whatever reason, the paperback has been popping up first for most people, which unfortunately I can't make free.
Assuming that you are, you have to click the "Buy now with 1-Click" button. It is definitely free, so you won't be charged anything. If you have any trouble, just let me know! :)