Quickstart
You have the project sources, you have the tool but then what?
Quickstart
Unzip the project archive to the location of your choice on your system, any path will work. Fire up the editor and open the project file DotAdditive.ngp from the folder you extracted the project into.
The project opens and the explorer fills up with the project content. At this point you might want to make sure everything is working as it should by launching the project. Select Execute... from the Project menu or by using the corresponding keyboard shortcut (default Ctrl+F5).
This is what you should see.
That's right, you can play the full game from the editor. Stop it at any time by pressing the Stop button in the toolbar or by pressing the corresponding shortcut (default Shift+F5).
Hack Around
Now that you confirmed the project to be working as it should you can start tweaking scripts and scenes to get a feel at how everything is tied together. Feel free to open a level scene (from the Scenes/ folder), tweak its components and test it right away. You do not need to test the whole project at once, most scenes can be previewed and tweaked directly via the Preview... entry of the Scene menu. Note that the scene menu is only available when the current tab is a scene view.
Be sure to check the other pages of this tutorial as they contain a lot of information on the structure of the project.