You can find plugins for rocketmod unturned at: https://hub.rocketmod.net/
Please stop your server before installing new plugins, and always reboot your server after a new plugin install.
First off, locate the plugin you would like to install onto your Unturned Server, download the files of the plugin and keep them for the next step.
Next you must locate the plugin installation folder on your unturned rocket mod installation, for Gameserverkings.com unturned servers, it is located at "/VServer/Rocket/Plugins/".
You then want to upload the files for the plugin into that folder (/VServer/Rocket/Plugins/), if the plugin is downloaded as a zip, extract the files from the zip, then upload the files created by the extraction.
(You can use either the file manager or FTP provided both free as a standard part of any GameServerKings.com service!)
Reboot your server, now the plugin will now be installed!