Thank you for the information. When I go to the site right now, I still don’t see the CSS or JS files for the plugin loading at all. You can see this for yourself if you open the Chrome dev tools and go to the Sources tab, where you’ll see a list of all the files loading on your site.
In fact, it appears that there aren’t any files loading from any plugin on your site. Not every plugin loads files on the front end of the site, but the jQuery Responsive Select Menu plugin does, and it requires them to load in order to work.
The selectors you mentioned look like they could work, so in order to keep investigating I need you to activate the plugin on your site and keep it activated while I look at the site. If we confirm that the plugin is active and the files still aren’t loading, then we likely have a problem further upstream, i.e. somewhere in your WordPress install, in which case there would probably be lines in your site’s PHP error log about the problem.