Filipe,
I did the exact stuff that the downloads told me to.
I extracted all of the addons into the addons folder from the .zip file.
Step 1) Download the ZIP
Step 2) Open the ZIP and see all the ZIP files inside
Step 3) Put all the ZIPS into your Blockland Add-ons folder (If you have Steam it's C: > Program Files (Regular or x86) > Steam > steamapps > common > Blockland > Add-ons) (If you have the non-Steam version it's Documents > Blockland > Add-ons)
Step 4) Once putting all the ZIPs into your Add-ons folder, open Blockland
Step 5) Select "Start a Game"
Step 6) Choose "Superkart" if you just want to play the gamemode, otherwise go to "Custom"
Step 7) Click every add-on that was included in the main ZIP folder of all your SuperKart related add-ons (You'll know they're enabled if the circle next to the add-on is filled)
Step 8) Click "Select" on the bottom-right corner
Step 9) Click "Launch Game" to start your server
PS If none of this works, I went into the best detail I could, then clearly you did something wrong or something is wrong with your game.