When you purchase a book with DRM, the download link in the confirmation email, or the Download button in your Feedbooks bookshelf, will give you a small .acsm file, which allows you to download your epub format ebook with Adobe drm. If you're using a PC or Mac, you'll need to open this file with Adobe Digital Editions, which will then automatically download your epub book. Once it's downloaded, you'll find your epub book in "My Documents > My Digital Editions".
If you're using a Sony T1 Reader, you'll need to use Sony's Reader Library software on your computer for your first download, and to transfer your book to your Reader via usb. This is to authorise the Reader to your account, for books with DRM. After doing this once with Reader Library, your Reader will be correctly authorised to your account, and you'll be able to download your books directly via wifi.
You'll find a full explanation and step-by-step illustrated guide for how to use Adobe Digital Editions (free download) to download and read books from Feedbooks here: http://www.feedbooks.com/help/adobe-de