You'd be better off not listening to Roland, his method is needlessly complicated. First off, I need to know what zip program you are using, the windows default acts like a folder in some ways, while a different program will open up and tell you what's what. You will only be able to unzip RAR files if you use a different zip program like
WinZip.
Windows default zip method: Goto your blockland folder (usually C:\Program Files\Blockland) Open the zip file you downloaded, inside the add-on zip, you should find a folder named "add-ons". Drag and drop this folder to your blockland folder, it will then pop up with a dialog (screenshot below), press "Yes to All". This won't delete anything and should ensure a proper installation. Done.
WinZip Method:Open the zip file, press extract, goto your Blockland install folder (usually C:\Program files\Blockland) then press ok, if needed, check your settings with the screenshot below. Finished.
If you are installing a script only (a .cs file for example) just download the file directly to your Add-ons folder.