All I had to do to make it work is edit the hardcoded path in
/Support/bin/add to point to my inbox.org file.
Another catch was that links to email messages in Org didn’t open Mailmate. The links weren’t linked. I added the following to my .spacemacs file and now everything works swimmingly.
(org-add-link-type "message" 'org-email-open) (defun org-email-open (record-location) (shell-command (concat "open \"message:" record-location "\"")))