The volume you're looking for is one of the published indexes that FindMyPast have in pdf form. I've never found them particularly easy to search, but once you've located the volume you want, it is browsable.
You need to search all records for the collection called "England & Wales Published Wills & Probate Indexes, 1300-1858", then by means of the search boxes find the volume you want. This will give you a list of pages in the book, each one a separate pdf file, and by clicking the Page Number heading you can put them in numerical order.
You'll need to pick the page you think most likely and browse from there. The navigation buttons only let you move one page at a time, so an alternative is to keep the listing page open, and open the pdf pages in new tabs. Another is by entering the URL directly, once you have a page open, as follows.
The Archdeaconry of Ely volume is published by the British Record Society (BRS), and the link to any page in their volumes ends in /BRS/xxx/yyyy, where xxx refers to the volume and yyyy is the image number. (In a browser address bar it might appear as %2fBRS%2fxxx%2fyyyy.) So to navigate quickly from one page to a distant one, you simply change the page number shown (yyyy) to the one you want to go to, and press Go or hit Enter.
I hope this helps you find what you want.
