From the main menu of the game go into the Level Editor, then go to the world selection screen from there. You will be presented with 12 different worlds to choose from, however if you click on the empty space in the bottom right corner (next to the sea/reef world icon) you will find that space to be selectable. Confirm your selection after clicking that space, and you will be able to design a level based around a tileset reminiscent of the graphics from the Apple II version of Lode Runner.