Okay, I decided to make a guide to using the map editor, since so many people are asking.
At the main menu, choose "design new world with parameters" and then press (e), set your custom variables (if you want a custom world to work, nullify all "minimum" values) and then press (p) to get to the painting stage.
You'll see on the left: a big window with hills all over it.
The right: Various values and items.
The bottom: Little keybindings and how big the brush is.
The window is where you will be painting your new world. Left click to paint, right click to scroll.
At the right hand side are the various things you'll be changing;
Elevation, Rainfall, Temperature, Savagery, Drainage, and Volcanic Activity.
Switch them all to "Not painting" by clicking "currently painting". Then, choose one to paint with by making it "currently painting", choose a value by clicking on the number (its default is 50) and then draw on the map. Tadaah!
Some things can be randomised; for instance you may want a constant temperature but everything else changed. To do this, click on the words "Will be saved" to have them randomise on gen.
When you're done making something really odd, press space twice, to take you back to the generation screen and press Enter to begin. Press 'y' if you don't want to save the changes.
Now the world will go crazy and try to generate. If it works, well done! If not, go back and make some changes that allow the world to be more livable (all mountain and desert with high savagery and temperature and no rain will not work. Then again, it might.).
You're done!
Perhaps somebody should put this in a sticky or on the wiki?