That will get you back to the start page of the site. There is a big box at the top of the page where you can enter the book's BCID (the special identification number that's listed on the label, which is usually inside the front cover). After that, the process should explain itself. Welcome and congratulations on finding a book!
"All the computer can give you is a manuscript. People don’t want to read manuscripts. They want to read books. Books smell good. They look good. You can press it to your bosom. You can carry it in your pocket."