The working directory may not be the same directory as the notebook directory for the notebook in which you're running the code. Use Directory[] to find out where your working directory is. Also, if SterographicProjection is a notebook, then it likely has the .nb extension, and NotebookOpen expects a full filename, including the extension. You could change your working directory, but personally I find it simpler to just create the file path:
NotebookOpen[FileNameJoin[{NotebookDirectory[], "StereographicProjection.nb"}]]
