The official Microsoft method for connecting Excel For Mac to a database is to use one of a few supported drivers, none of which are open source:
https://support.office.com/en-us/article/ODBC-drivers-that-are-compatible-with-Excel-for-Mac-9FA6BC7F-D19E-4F7F-9BE4-92E85C77D712
Personally, I've used the Actual Technologies driver with Excel For Mac 2011 and it worked, but I had problems with it with the 2016 version of Excel. I was using Postgres as well.
Instead I decided to write the data from the DB out to a csv, and then connect to the csv from Excel. That way I don't have to deal with the database connection, but it's not ideal.