Your brain.
Your hand.
Your index finger.
Your mouse.
Click on the blue link to the add-on. You might have a message pop up and ask you if you want to extract the folder. Click No/Cancel. Then open the folder the add-on was put into. Then, open another window and navigate to your Add-on directory.
For windows users it is in "C:\Program Files\Blockland\Add-Ons, for Mac it is "HD\Users\Yourname\Applications\Blockland\Add-Ons"
Then, click on the window with your add-on, then click once on your add-on and hold down the mouse button. Drag the cursor/add-on file to the other window, and release the mouse button.
Then open up Blockland by clicking on the big black B. Click "Start Game" then click "Add-Ons" and scroll down until you see the add-on and click on the box next to it. Click "Done" and then click "Start Game" again and voila. You're all done. Enjoy your new add-on.