The mission editor is now included when you install BL.
Here's what you need to do. Find the shortcut icon on your desktop...
1. Right-click on the icon and hit Copy
2. Right-Click somewhere else on the desktop and hit Paste
3. Right-click on the icon and hit Properties
4. Find the box labeled Target:
5. You'll see something like C:/programfiles/blockland/blockland.exe
6. At the end of that line (inside the text box) add a space and then type in -mod editor
7. Now hit the OK button at the bottom of the window and it should close
8. Double click the new icon you made and it will open Blockland in editor mode.
Note: it won't look any different from the normal Blockland menu. You will know you are in editor mode when you start a map and hit the F11 key. Good luck from there.
Bonus step: Right-click the new icon, hit Rename, and type in a new name like "Blockland Editor".