Restart to load. Mods are only picked up at server start — adding one to a running server does nothing until you restart.
Players auto-download required mods when they connect, so they don't install anything manually.
Match versions & dependencies. Use the mod build for your server's version, and check the mod page for any required dependency mods.
Troubleshooting: if the server won't start after adding a mod, remove it from mods, start to confirm it was the cause, then re-add the correct version.