The billing address field is on the Billing page under the "Billing Info" section, where you can click "Update billing details" or "Add Billing Address" to open the address form.

That option doesn't exist for me:

Sorry you are having problems. I can see you are active on a Hobby plan now, so it looks like you resolved the issue. If we've got that wrong somehow, can you try in a different browser or do a hard page refresh? We made a recent update and have had reports from some customers that their browser was caching the old page.