On your top toolbar, click on the "Go" tab and all the while hold down the left alt button
A new tab labeled "Library" will appear and you have to click on that and navigate to the "Application Support" folder.
Then scroll all the way down till you see Steam, then SteamApps, then common, then Blockland. That's how you find your add-ons folder. Put any downloaded add-ons into that folder.