As explained on our website, our MoodleCloud School sites (Mini, Small and Medium) have a number of selected plugins pre-installed. Check out what plugins are included in our MoodleCloud for School sites here:
https://moodle.org/plugins/browse.php?list=set&id=80
To keep the platform simple and stable for all users we do not allow you to install your own plugins at this time, so if you require a particular plugin to be installed immediately for your MoodleCloud site, we can pass you onto your Moodle Partners to discuss your custom needs. Please note that if you want to have plugins added to your site, you will need to move to a Moodle site hosted by our Moodle Partners or a self-hosted site.
Warm regards,