I am familiar with object navigation and that was how I was trying to access the list, using NVDA + num pad 4 and 6 to move up and down it.
I gave up and called in sighted assistance this afternoon, and finally figured out why I was having trouble with the month a and year lists .... there were no items in those lists.
What I hadn't realised was that in the address fields it thought that my Microsoft region was Turkmenistan! Presumably they don't use credit cards in Turkmenistan! When I changed my region in my Windows settings and re-visited the store page, I did not have to enter any details other than choosing the buy now button. I assumed it was pulling in the card info I entered into my Microsoft account yesterday, but no — it automatically used my PayPal info which I'd used to purchase MS Office 2016.
So thanks Gene and all other respondents on this question — I now have KNFB Reader installed and am playing around with OCRing of some PDF documents :)