Download files from the "Add-ons" subforum, then find the folder called "Add-ons" in your Blockland forum.
Place the files you downloaded into that folder.
Start up Blockland, go into "Start Game", and click the "Add-ons" button under the map list.
Check off the Add-ons you want to use.
Start a game.
Any more questions should be answered by reading this topic:
http://forum.blockland.us/index.php?topic=40560.0