Using the left menu, click Websites, then click on the website you have WordPress installed on.
Under the Applications installed heading, click manage, here you will find all your WordPress website options and settings.
Using the inset left menu, click Plugins. Here you can see the default installed plugins, there installation status, you can delete them, upgrade them and also enable auto-updates.
To install a new plugin
Click the install button, either use the Featured, Popular and Top Rated tabs, or use the search icon to find specific WordPress plugins.
Find the plugin(s) you would like to install, click on it so it shows as ticked and click the install button.