What are the most important websites where university jobs in Math Education are advertised? The kind of boards I am looking for should preferably be able to reach an international audience.
The main place of which I am aware is the math education listserv run by Dr. Jerry Becker.
I suggest that you contact him to ask about listing a position; he will send it out with a subject line that begins "Position Announcement"; early September is not the most common time to list a position, but here is a screenshot of his listings from 27 Oct 2015 - 18 Dec 2015 to give you a sample glimpse:
(Click through to see an enlarged image.)
I suggested as a side-note back in MESE 8193 that math educators should also subscribe to Dr. Becker's listserv; I reiterate that suggestion here!
In the U.S., MathJobs.org dominates.
Mathjobs.org is an automated job application system, sponsored by the American Mathematical Society. We serve all job applicants with advanced degrees in Mathematics, and employers (academic or industrial) searching for mathematicians. The system is free for applicants.
Despite its U.S. focus, they are international to some extent:
(Image from www.mathjobs.org.)
I used the Chronicle of Higher Education when I looked for jobs. Is it still useful? (They now have a special site for jobs, chroniclevitae.com.)
Although MathJobs was mentioned, there is a larger list (or perhaps it was a larger list) at the Employment Information in the Mathematical Sciences website, also by the AMS. It is not an application site, though (as opposed to MJ), and I don't know how many math ed jobs there would be - today it was everything from OR to stats to 'math curriculum designer' (whatever that is) to various postdocs.