Dragging and droping into the add-ons folder is the way no matter what operating system you're on.
It doesn't magically change if you're on a different OS.
Jalen, there could be serveral issues:
You left Blockland open when you downloaded the add-on. Solution: Restart Blockland.
You downloaded a failed add-on. Just delete it.
You installed Blockland into program files. That wouldn't cause the add-ons to not work, but you should install it to just C:\ anyways when on Vista.