Hi @radjr,
I recommend fixing the Plugin Gallery (see OJS 3.1.1 Plugin Gallery not loading - #6 by asmecher); there are lots of pitfalls in manually installing plugins that the Plugin Gallery will take care of, such as ensuring the plugin is compatible with your version of OJS, and taking care of various installation steps that are easy to forget if you’re installing manually.
If you’re curious where the Plugin Gallery gets its data, it’s http://pkp.sfu.ca/ojs/xml/plugins.xml; there’s not currently a “user-friendly” view of this outside of OJS, though.
Regards,
Alec Smecher
Public Knowledge Project Team