Ok step by step then.
1. When you've found the add-on you want click on the download link.
2. A window will appear with the options "Open" "Save" and "Cansel".
3. Press the "Save" button and go into "Blockland" folder and then into "Add-ons" folder.
4. Press "Save" button in the right corner of the screen and start Blockland and wait for the computer to download it. Then you can just close the window. Now start Blockland.
5. Go to "Start a game" and then click on the "Add-ons" button.
6. Find the Add-on you just downloaded (it will be unabled) an enable it.
7. Start the game and see if it works.