Let me clear somethings up...
What do you mean by installed? Do you mean you put the .zip folder into somewhere, or something else? If you put the .zip somewhere, where did you put it? Your post makes it seem like you put it in the Blockland folder itself, not Blockland/Add-Ons
When you said you tried opening the Add-Ons folder on the browse>download, what is this browse thing?