I was wondering how can I install the plugin Mathjax which enable Math symbols on OJS. Recently some one made the plugin in the following link but he did not explain how it has to install on OJS. https://github.com/vasylOstrovskyi/mathJax
Could be that you have to run php tools/upgrade.php upgrade for the plugin to be registered properly. After that you could check if you can see the mathJax scripts being called in the source code. If they are called, then the problem could be with the allowed_html settings in your config.inc.php. I am not a mathematician so I do not recall what the markup actually looks like. But it could get filtered out with the default allowed_html settings.
or you could create a theme or a child theme and add the edited templates there and they would override the default templates
or you could create a theme or a child theme and add the javascript there
The first one is problematic because you need to do that after each OJS upgrade. The two other options will require more work and some knowledge about creating themes in OJS, see https://pkp.gitbooks.io/pkp-theming-guide/content/en/
But as I said above, I am not sure if this is enough. OJS has a setting in config.inc.php for allowed html tags and it will remove all other tags from for example the abstract. So you will probably have to deal with as well. Also, I am not sure how the tinymce editor OJS is using for text editing supports math. So make sure you start with something simple.
I saw that you asked the plugin author about this so maybe better to wait for his answer, or you could try adding the script to the places I mentioned above just to see if you can get math working.