Open one of the .zip files in your Add-Ons folder with your file browser, an archive manager or Windows Explorer on Windows XP/later.
Add-Ons are packaged in .zip archives saved directly to the Add-Ons folder - they must include a description.txt file for referencing and either a server.cs or client.cs file. (or both for some mods)
server.cs is executed when you start a server, if the Add-On is enabled. Scripts, weapons, vehicles, sounds, bricks and player types are run here.
client.cs will always execute when you start the client (window, non-dedicated server) copy of Blockland. Use this for GUIs and other such things.