If you cannot see the plugin manager button then please ensure that CURL is enabled on your server.
The Plugin Manager's purpose is to allow users to download and install all optional plugins currently available to Jomres right in the component's control panel. This makes it easier for us to introduce new plugins and deploy upgrades, and for you to find new and upgraded plugins.
The manager can download and install several different types of Jomres plugins: Internal plugins Minicomponents Modules (CMS specific, like search modules for Joomla 3.x) Plugins (CMS specific, like bot_jomres_bookingform for Joomla). If a plugin is a CMS specific plugin, like a Joomla Module, then you will need to use Joomla's Discover feature to complete the installation of that module.
The Plugin Manager will list all currently available plugins. You can use it then to install/upgrade or remove the plugins of your choice, however some plugins like Modules, Plugins and Minicomponents can only be configured and uninstalled via the standard Joomla configuration tools. All plugins can be upgraded direct from the Plugin Manager. Once you have installed a plugin, you will be able to use it until you uninstall it again. Be aware, however, that if you uninstall a plugin after your Download and Support license has expired, you will not be able to download it again.