The SiN Episodes Software Development Kit is now available in the Tools tab of the Steam client. Among other things, it comes with a level editor, allowing you to create new single player levels and Arena Mode maps. Three source map files are also included, so you can check them out in the editor and see how Ritual did some of the things in the game.
As noted by Michael Russell in our forums, you should run SiN Episodes once after downloading the SDK, in order to allow the game to extract all the files necessary for the SDK to work correctly.
Related, the Ritual Editing Community Wiki has also gone live, offering editing information and tutorials for SiN Episodes, Star Trek Elite Force II and Heavy Metal FAKK2. Anyone can add information to the Wiki, so register your free account now and get editing!