That page should have a list of books you own. On the left side of each row for each book, there should be a light gray button that looks like: [...] It's u

That page should have a list of books you own. On the left side of each row for each book, there should be a light gray button that looks like: [...] It's under the "Actions" column, second to the left. Click that and there should be a popup that contains a link "Download & transfer via USB" I believe this link will only be available if your account has a real kindle (e.g. eink or fire) registered to it. It encrypts the downloaded file for whichever registered kindle you select from a list of all your registered kindles and apps, with the apps grayed out so you can only select actual kindles.