All you need to do is provide Friendica with the base URL of MathJax. This can be either the URL of the CDN of MathJax or your own installation.
In case you want to use the CDN you can try the following URL as a quick start
In case you don't want or can use the admin panel of Friendica you can activate
the addon by adding mathjax to the list in your
'system' => [ ... 'addon' => '...,mathjax' ... ]
and then providing the base URL after that in the
'mathjax' => [ 'baseurl' => '[the URL to your MathJax installation]', ],
Once the addon is configured you can use LaTeX syntax in your postings to share formulae with your contacts. But remember that the formulae are rendered in the browser of the user thus your contacts need to activate this addon as well. If they don't they will only see the LaTeX syntax in your texts.
Just enclose your equations in $$...$$ pairs like e.g. $$f_c(x)=ax+b$$.