I know you said you are putting it in the add-ons folder under documents, but please make sure you are. There have been a lot of help topics about add-ons not working because of placing them in the wrong add-ons folder.
Also make sure that Blockland is not up when you are putting the add-on into the add-ons folder. The add-on only appears when you put it into the folder, then open Blockland.
Another thing: make sure you are looking in the add-on list in the "Start a Game" menu. If you download it from the forums, then it won't be in the list of add-ons in RTB, unless you hit "Non-RTB Add-ons" (or whatever) at the bottom.
