It took me two days to find this out. I wanted to go to the page that contained books starting with, for example, authors in my Cloud who had a last name starting with "s". The first Amazon rep said it couldn't be done on the Paperwhite, as we had been able to do on the Keyboard Kindle. The Kindle User's Guide (Loc 191) implied I could by "tapping the x/y on the Home page." I had no idea what this meant. The second call got me Julio who spent some time researching it.
The "x/y" turns out to be the page number location in the lower right of the screen on the Home Page.
Go to Home and tap Cloud. You'll get the first page of your archive listed alphabetically by author/title (your choice.) At the bottom right you'll get the number 1/(number of pages in your archive.) Mine says 1/65. Tap that number and a new screen will appear that allows you to go to a particular page in your archive or, what I was trying to do, the page where the author/title starts with your letter of choice. When I type "s" and tap Go it sends me to page 51/65 where the first authors that start with "s" reside.
The pages are predetermined by the number of books you own so don't expect the "s" target to be at the top of your first page. More likely a few "r" entries will begin that page but this is no problem. Just sweep to the left to go to additional books/authors in your search.