Shafin's guide on how to re-install your lost default add-ons:
Step 1: Install blockland again.
Step 2: Save the new blockland file somewhere else.
Step 3: Open the new blockland file.
Step 4: Head to the Add-ons folder.
Step 5: Copy the folder that you are missing.
Step 6: Paste it into your original blockland add-ons folder.
Step 7: Open Blockland.
Step 8: ???
Step 9: Profit.