So, I tried to look at and then set up some DNS records at FM, following the instructions at
https://www.fastmail.com/help/receive/dns.html, where it refers to the Settings-->Domains page, click the "edit" button by my domain and then follow the "Show DNS settings link." Only there is no such link on the Settings-->Domains page that I can see!
If I go back to the "Settings-->Domain" page and follow the "add domain" button I see that there is a "Show DNS Settings" link right above the beginning of the MAIL section, but that link does not appear on the page that I get to from Settings-->Domains and then click the "Edit" button next to my one and only domain.
Am I missing something obvious?