What do you mean by "is within Sweet Home 3D" ?

within "Sweet Home 3D Viewer" applet and nowhere else. It's the file name of the document this applet is trying to open by default. Why are you trying to run the applet from Eclipse?

From where exactly the program try to load this default file ?

It's in ViewerHelper class which is actually used only in Sweet Home 3D Viewer applet.