When you have downloaded the Add-on that you have chosen open
Computer/OC (C:)/Program Files (x86)/Steam/steamapps/common/Blockland/Add-Ons and place the add-on that you have downloaded in add-ons folder. That is it
You can enable Steam to work with Blockland by clicking the link below. It's easy but make sure you make a backup of all your saves and add-ons
https://blockland.us/steam-redeem.php
Once an add-on is installed, you will have to enable if you want to use it in-game.
To enable an add-on:
* Start Blockland
* Click "Start A Game"
* Select the "Custom" Game Mode
* Click "Select"
* Check the Add-Ons that you want to use
* Click "Select" again
If you already knew this then move on.