To function as a Blockland Add-On, your zip file will need at least:
description.txt file (Correctly formatted with Title and Author fields)
either a server.cs or client.cs file
If an Add-On has a server.cs file, it will appear in the Add-Ons menu and be executed when enabled and starting a server.
If an Add-On has a client.cs file, it will only execute once when you start the game. (In non-dedicated server mode)