@the OP: there's also an "insert code" option below the register now button. Otherwise try the upper "upgrade" button, there may be a way to insert the code there too in the popup dialog. I don't have any "free version" available now so I'm just guessing.
But anyway, you'd be better off downloading an AIS trial version and insert the code there... as I have no idea what happens when you do that with an AIS code entered in the "free version", if additional components get automatically unlocked/downloaded or not...