Right click on the Blockland short-cut then this will pop up:

Click "Properties"
Then this should come up:

Then add " -mod editor" Without the " at the ending of the Target. should look like this:

Then click "Apply" then you're done.
EDIT: there has been people wanting to know how to open it after they are done.. Just go in-game then click "F10" or "F11" F11 is the mission editor and F10 is a gui Editor/Creator.