The mission editor comes with your game, but it isn't enabled by default. Change the shortcut to Blockland (if you have one) by editing the properties and adding "-mod editor" after the Target path.
If you happen to use Windows:
On Windows Vista, it should look like this:
C:\...whatever path to the file you have...\Blockland\Blockland.exe -mod editor
On Windows XP, it should look like this:
"C:\...whatever path to the file you have...\Blockland\Blockland.exe" -mod editor