I know what your doing wrong. Your putting the components from System_ReturnToBlockland.zip file, into your add-ons folder.
When you download, you click "Open File Location". Don't just "Open" the file.
Click "Open File Location" when you download RTB. Now go to Desktop, right click the Blockland Icon. Then Click "Properties". Then "Open File Location". BOOM, now you know where the add-ons folder is.
Now move System_ReturnToBlockland.zip into Add-ons.