Watermarked ebooks are delivered in their original format. One can read watermarked ebooks with any software or device supporting the file format indicated in the book information page.
Go to Adobe's pages and make yourself an Adobe account. Please note that your Adobe ID is the e-mail address you're using when registering.
Install Bluefire Reader from Appstore and authorize the software with your Adobe ID.
If needed, please see more instructions about installation from here.