You install MetaMod Source and Sourcemod into Dedicated Server (SRCDS).
- Use SteamCMD to install Source Dedicated Server (L4D2)
Addons do not support listen server (server created through in-game).
Thank you very much for the reply, the problem has now been solved. Forgive my stupidity, it was just because I forgot to remove srcds.exe's startup parameter "-insecure"