When you download add-ons off of another server, you only download the model, textures, sound, etc.You don't download the scripts, therefore you can't use it.If you want add-ons, get them off the add-ons section.
Though I know very well that I can go download them myself off here, why can't I get them off the game? Games like Empire at War let you load custom maps and crap like that from joining a match that includes them, so why not BL?