I partially solved my own problem, posted here to help others. The DMM Help file seems to be incorrect about including an Account field, or at least incomplete.
In my Accounts query (that generates the Accounts.csv), I pick up the primary source address as Address1, and name it 'Primary'. Accounts.CompanyName acts as my unique identifier for each Account. In my Addresses query, I pick up all non-primary source addresses along with the Company Name. When mapping Addresses to the Address (customeraddress) entity, I set my Company Name column to the Parent attribute. I'm then given a choice of Account or Contact (all of my addresses are companies), I select Account Name* from Account and the import proceeds.
There must be no duplicates in the Accounts.Account Name attribute.
