This is so easy.
Find your Blockland shortcut(see method 2 for another way)
The shortcut will often be on your default as Blockland has probably installed.

Add the following arguments, "-mod editor".

Option 2
start blockland.exe -mod editor
Make a file called anything.bat, in BL's main folder
Now shortcut to it or simply click on anything.bat
Now what? Now you start your own server(LAN or internet servers only), then you press F11.
For the GUI editor, you press F10. The GUI editor will work whenever Blockland is open, however.
In order to close the GUI editor correctly, you must switch the GUI back to MainMenu or playGUI, and then press F10. If you don't do that, your game will be messed up.
As you can see, they are executed before the game starts in a section called "LOADING MODS". The editor is located in editor.zip which is found in the Blockland folder.