For instructions on how to play, go here.
The level editor is now available! To use it, just download the file to the folder of your choice and run it. There are some brief instructions in the Help menu of the editor to get you started. One thing to note is that choosing backgrounds does NOT change the background in the editor window. When you open your level in the game, however, you will see your selection.
Levels created with it can be loaded up in the game by choosing the Custom Stage option in the Stage Select menu. You can even select multiple levels at a time!
If the game isn't working for you, you may need to upgrade your Java software. If so, go to the download page and follow the instructions. The game should automatically launch when installation is complete.