Ok. when you join the server with addons you dont have in it, it just downloads any textures, sound files, or shapes(models) that it would need inorder run properly. The only missing part is the script which is a text based document and doesnt download from the server. In order to actualy run the mods on your own server you would need to download the file from the fourm, website, or the RTB mod manager if you have RTB.(RTB is a mod that gives players some new fetures to blockland the the I.R.C.{Internet Relay Chat})
Hope this helps you out.