Add-Ons are distributed as zip files. To install them, simply save the zip file to My Documents/Blockland/Add-Ons (windows) or /Users/<username>/Documents/Blockland/Add-Ons (macintosh). Do not extract the zip file.
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