As of right now, Sequel Ace can only connect to a MySQL database over TCP/IP (ie, a networking port) and not over a Unix socket.
Windows machines already connect to the DB over TCP/IP because they don’t have unix sockets, but if you want to change a site under MacOS to use TCP/IP, then you can follow the steps outlined in this FAQ:
Once the site has been updated, then you can connect to that database in whatever way your SQL client uses to connect over TCP/IP