Depends on what addon you want to make...
You can get Addons from either: -Blockland Fourm's Add-On Page -Return To Blockland
you go to the blockland forums like here: http://forum.blockland.us/index.php?
return to blockland is an free version of blockland but it lags by bunch of bricks and there you can see more about torque engine and normal blockland has more like addons and more maps and official website but return to blockland has only bunch of Lego weapons and items and Lego bricks! here isn't physics and there is multiplayer! you can save things and load it again.
find one of the blockland sites
In the demo you can't create or join servers, load more than 5 addons, or place more than 150 bricks.
No.
Make sure your driver is up to date. Try to ask help from the people of the Blockland Forums.
Byuilding and eventing one.
You have to go to Return To Blockland to download stuff for Blockland.
Blockland generate key online: infiles.org/blockland/
You go to Return To Blockland (RTB) where you can download anything for Blockland.
They are addons for a specific class that you play. I play a priest so I get addons specifically for priests to make it easier for me to play my class.