If by "root folder" you mean the root of your s9y installation, nope. There's a directory /plugins/, so the extracted plugin directory would have to go into that, for instance /plugins/serendipity_event_cal/ (including all files). After that, you should see it in the list of installable plugins, so under "Plugins" in the backend, it's either "Install a new sidebar plugin" or "Install a new event plugin". Depending on, obviously, what type of plugin you want to install.
if you want to make this easier on you (trust me, you want to), you should install the Spartacus plugin included in the serendipity core (so you should already be able to install it as a new event plugin). That will give you the ability to install and update plugins and themes from your s9y backend without having to upload them manually.