You have full file access to your server, through the browser file manager for quick edits and SFTP for anything bulky.
Installing
- Stop the server. Adding files to a running server produces inconsistent results.
- Upload the plugin or mod file to the correct directory for your platform.
- Install any dependencies the author lists — a missing library is the most common install failure.
- Start the server and watch the console for load confirmation or errors.
Plugins versus mods
Plugins run server-side only and players connect with an unmodified game client. Mods generally need matching versions installed by every player as well as the server. Check which one you are dealing with before you promise your community anything.
Add one thing at a time. If you install six plugins at once and the server will not start, you have six suspects instead of one.