The help page on including js file for newbies, must be outdated, because it says to put your "<script>..." line in your header.php. What header.php? Then the simple task of finding my header.php or equivalent becomes a nightmare. I'm not interested in knowing the inner workings of WP at the moment - I just want to include a js file. Like so:
<script type="text/javascript"
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>
Grazie.