Transferring your epub to an iPad (or iPod, or iPhone):
Once you’ve downloaded the book, find it on your hard drive.
Drag the ebook file to the tray at the left hand of the iTunes window. Aiming for the “Books” icon will do you well, though the whole section will work, too. This will automatically add the ebook to your iTunes library. Click the Book menu to make sure this has happened.
Sync your iPad/iPhone/iPod. Depending on your sync settings, the ebook may automatically be added to your iPad. If not, you may have to go to the “Books” tab in the iPad management screen and check the box next to the new book.
Once the ebook is synced to your iPad, read it by going to the iBooks app and tapping on the icon for the new book on the bookshelf.